* Update .NET SDK to 6.0.100-preview.1.21077.13

* Update VS version to 16.8

* Update C++/CLI projects to v6.0

* Update managed projects from .NET 5.0 to .NET 6.0

* Update C++/CLI projects to .NET 6.0

* Update multitargeting.props to .NET 6.0

* Update WpfArcadeSdk to .NET 6.0.  Fix intellisense issue.

* Update WpfSdk v5.0 to v6.0.  Fix WinForms accessibility reference.

* Suppress validate platform compatibility.  WPF only builds for Windows.

* Update packages to .NET 6.0

[master] Update dependencies from dotnet/winforms
- Coherency Updates:
  - Microsoft.Win32.Registry: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.CodeDom: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Configuration.ConfigurationManager: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Diagnostics.EventLog: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.DirectoryServices: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Drawing.Common: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Reflection.MetadataLoadContext: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Security.AccessControl: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Security.Cryptography.Xml: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Security.Permissions: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Security.Principal.Windows: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Windows.Extensions: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - Microsoft.NETCore.Platforms: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.IO.Packaging: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - Microsoft.NETCore.ILDAsm: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - Microsoft.NETCore.ILAsm: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - System.Resources.Extensions: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - Microsoft.NETCore.App.Internal: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - Microsoft.NETCore.App.Ref: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)
  - Microsoft.NETCore.App.Runtime.win-x64: from 6.0.0-alpha.1.20428.2 to 6.0.0-alpha.1.20515.8 (parent: Microsoft.Private.Winforms)

 - .NET Core WPF Build error on custom BaseIntermediateOutputPath #1718 (#3120)

* Use more robust relative path calculation in markup code generation

 - Do not reflow lines after measure

 - disconnect HostVisual on its own thread

 - Fix three scrolling hangs

 - remove useless Assert

 - handle re-entrant request to close ToolTip

 - Move StackTrace to error branch

 - FixedPage SOM bugs

 - Don't add null item peers

 - Use correct lock object for predefined packages

 - repair bad copy/paste

 - fix build break

 - remove AppContext switch

 - DataGrid.Copy: fail silently if clipboard is locked

 - update_intellisense_artifacts

 - Remove or replace Policheck violations in code comments (#3606)

 - Merge pull request #3542 from dotnet/rc2-askmode-BaseIntermediateOutputPath

Ask-Mode: [release/5.0-rc2] Custom intermediate output paths shouldn't break markup compilation

 - Merge pull request #3564 from SamBent/VSPFreeze

Vsp freeze

 - Merge pull request #3565 from SamBent/AutomationPeerNull

Avoid null item AutomationPeers

 - Merge pull request #3566 from SamBent/TextReflow

Avoid reflow of shaped text

 - Merge pull request #3567 from SamBent/HostVisualThreading

HostVisual threading

 - Merge pull request #3568 from SamBent/PopupReentrancy

Reentrancy when closing ToolTip

 - Merge pull request #3570 from SamBent/ClearTypeAntiAliasing

ClearType anti-aliasing

 - Merge pull request #3574 from SamBent/NoisyStackTrace

Move StackTrace to error branch

 - Merge pull request #3575 from SamBent/FixedPageOverlap

FixedPage SOM bugs

 - Merge pull request #3576 from SamBent/LockedClipboard

DataGrid.Copy: fail silently if clipboard is locked

 - Merge pull request #3577 from SamBent/PredefinedPackageSynch

App resource threading issue

 - Revert "Merge pull request #3542 from dotnet/rc2-askmode-BaseIntermediateOutputPath"

This reverts commit 3d4e91cb, reversing
changes made to ab840c47.

 - Merge branch 'release/5.0' of https://github.com/dotnet/wpf into release/5.0

 - update intellisense version

 - missed a version

 - 1228498 [ wpf ][ PoliCheck ] - Defect : Term "nuked"

 - Merge pull request #3637 from dotnet/revertPath

[release/5.0] Revert path change to markup compiler

 - Merge pull request #3642 from dotnet/policheck2

1228498 [ wpf ][ PoliCheck ] - Defect : Term "nuked"

 - Update arcade

 - Merge remote-tracking branch 'upstream/release/5.0' into darc-master-f9327a78-0358-4132-8625-923a71a6b121

 - Update wpf-int dependencies

 - Replace null comparisons on non-nullable types that now cause compilation errors, due to the .NET SDK update.

 - PR feedback

 - PR feedback

 - Merge pull request #3640 from AdamYoblick/release/5.0

Update intellisense version

 - Merge remote-tracking branch 'upstream/release/5.0' into darc-master-f9327a78-0358-4132-8625-923a71a6b121

 - Merge branch 'darc-master-f9327a78-0358-4132-8625-923a71a6b121' of https://github.com/dotnet/wpf into darc-master-f9327a78-0358-4132-8625-923a71a6b121

 - Suppress obsolete API warnings. These must be fixed for 6.0.0.

 - Merge pull request #3659 from dotnet/fixdarcupdate

Temporarily suppress obsolete API errors to get WPF master building

 - Merge branch 'master' into darc-master-f9327a78-0358-4132-8625-923a71a6b121

* Update dotnet-api-docs_netcoreapp3.0 to v2 which contains WindowsBase intellisense files

* Update dotnet-api-docs_netcoreapp3.0 to v2 which contains WindowsBase intellisense files

* Update dotnet-api-docs_netcoreapp3.0 to v2 which contains WindowsBase intellisense files

* Update dotnet-api-docs_netcoreapp3.0 to v2 which contains WindowsBase intellisense files

* adding other assemblies

* removing unknown character

* adding apicompat baselines

* updating themes projects and licenses

* fixing markup compilation

* addign PresentationUI

* adding WindowsFormsIntegration

* updating solution to add WindowsFormsIntegration

* add WindowsFormsIntegration-ref project to sln

* adding PresentationUI ref project

* reverting line ending change

* adding comment to describe changes to pbt.targets

* fixing solution

* adding PresentationUI-ref to sln

* adding deploy configs for System.Printing and DirectWriteForwarder

* undo change to make everyone happy

* Couple of conditions were wrong in two targets, preventing them from running correctly and calculating NetCoreReference values during reference-evaluation. Fixing the conditions fixes the evaluation of NetCoreReferences during builds.

* Adding DARC dependency for Microsoft.DotNet.GenAPI and updating

* Version updates, adding basic target

* Enabling GenAPI to generate sources for all applicable assemblies.
Disabling use of ProduceReferenceAssembly across the codebase.

* Adding PBT to GenAPI project list.

* Initial build targets for GenAPI and generated reference assembly projects.

* Reverting to projects explicitly declaring their needed PackageReferences.  Otherwise we can potentially import at incorrect times, overriding various values from the WpfArcadeSdk.

* Adding additional GenAPI params.

* Switching to generate the reference assembly project via a task using the runtime assembly's project as a template.  Based on PBT's GenerateTemporaryTargetAssembly.

* Adding project exclusions for WIndowsBase

* Turning off XLIFF generation for reference assemblies.
Removing EmbeddedResources from reference assemblies.

* Removing module initializer injection from PCore-ref

* Removing CompileDependsOn from PFramework-ref

* Removing disabling of default compile items to pickup the generated reference sources.

* Adding global API exclusions file.

* Removing further default excludes.

* Adding initial exclusions and NoWarns based on building.

* Initial change to manual reference assembly projects.
File dumping the initial generated code and projects.

* Modifications to build System.Xaml-ref.
Changes to ensure that reference assembly projects are properly bin placed into the ref pack.

* Fixing issues with checks against hand ref assembly property.
Adding WindowsBase reference assembly.

* Adding UIAutomationTypes reference assembly

* Adding reference project for UIAutomationProvider

* Adding UIAutomationClient ref project

* Removing workaround for WindowsBase GenAPI issue and adding back manually fixed output for WindowsBase reference assembly.

* Adding UIAutomationClientSideProviders reference project.

* Adding System.Windows.Input.Manipulations reference project

* Adding PresentationBuildTasks reference project

* Adding PresentationCore ref project.

* Cleaning up PresentationCore ref assembly and adding exclusion notes.

* Adding security attributes to exclusions

* Fixing exclusion list

* Fixing exclude param

* Fixing attribute exclusions to match CoreFX.
Fixing cmd line params to GenAPI.

* Applying new exclusions to ref assembly code and fixups

* Cleaning UIA assemblies and updating code with latest exclusions.

* New exclusions and cleanup

* PBT cleanup and new exclusions

* New exclusions and fixups for PCore ref assembly

* Adding ReachFramework ref project and cleaning.

* Adding PresentationFramework reference assembly and fixups.

* Removing any extraneous ProjectReferences from reference assembly projects.  Anything that is not itself a reference assembly was removed.

* Adding back assembly attributes to ref assemblies where applicable.
Fixing PresentationFramework-ref to use private copy of asm attributes due to internals dependency.

* Removing PBT reference assembly as we're not shipping one anymore.

* Further removing PBT-ref

* Adding the ReferenceAssemblyAttribute to all hand-crafted ref assemblies.

* Removing no longer applicable tasks

* Fixing an issue with incorrect signing of System.Xaml.

* Enable ApiCompat against new ref assemblies.
Fixing ApiCompat issues with PCore and re-baselining.

* PresentationFramework ApiCompat baseline

* ReachFramework ApiCompat baseline

* System.Windows.Input.Manipulations ApiCompat baseline

* UIA ApiCompat baselines

* Fixing removal of ObsoleteAttribute and ApiCompat baseline for System.Xaml

* ApiCompat baselines for WindowsBase

* Updating WindowsBase ApiCompat baseline.

* Adding back attributes that were removed incorrectly.

* Fixing missing attributes

* Fixing missing attributes on reachframework

* Updating PCore baselines and adding back missing attrs

* PresentationFramework fixup + baselines

* Fixing ApiCompat bugs

* Re-baselining with ApiCompat fixes and removing old, no longer applicable baseline files.

* Updating reference assemblies due to System.Security.Permissions changes.

* Fixing up ResolveMicrosoftDotNetWpfGitHubReferences so that dotnet_wpf_int references lib assemblies instead of ref assemblies from the GitHub package.

* Fixing up ApiCompat for C++\CLI vs C# runtime projects.

* Fixing PackageRefernece for RID specific MicrosoftDotNetWpfGitHubPackageso that it doesn't get added when the package isn't defined (building in the open).

* Adding dotnet-wpf-int ref assemblies to signing and apicompat.

* Cleaning up reference assembly generation.
Fixing Intellisense XML placement.

* Add back internals to support XAML compilation.

Preview 4 branch snap into release/3.0 

This PR onboards a new shared build infrastrucutre - primarily props + targets - that are shared between this repo and WPF's internal build.