* Significantly improved project folder support

Fixes #2178

* Better support for non-Windows style directory separators in .fsproj

* Enable support for "Add folder" on project node

* Fix copy/pasting files within subfolders

Fixes #2048

* "Add existing" on the project node now places the file in the correct subfolder

* "Add existing..." now creates any necessary intermediate directories

* Improved resilience of Add Above/Add Below with folders

* Localised new error messages

* Add "New Folder" to Add Above/Add Below

* Updated FileCannotBePlacedMultipleFiles localisation to be more user-friendly

* Fixed being unable to "Add existing" on files outside of the project hierarchy

* Rename AllChildren to AllDescendants

* Fix cut/paste folders resulting in an infinite loop

https://mpfproj10.codeplex.com/workitem/11618

* Fix test compile error

* Make InsertionLocation internal

* Add support for linked files

"Add Existing" on files outside of the project hierarchy will now copy them to the target node

* If the folder exists on disk use that instead

* Do not delete linked files when deleting folders

* Remove bogus test - it's not possible to add a file to the project that has the same name as an existing file