* Fix remaining ftl->fxl conversions

The previous scripting pass at this did not account for objective c file endings

* Update tonic DEPS reference to the post-fxl version

* Various iOS a11y fixes

* undo

* review comments

* review comments

* Adding SemanticsFlag.isSelected

* typo

* Retain the parent of a SemanticsObject

This fixes the last known crash in iOS accessibility.

* rework memory management

* review comments

Previously, taps were sent to the topmost tap handler in the center of the accessibility frame of the selected element nd therefore might have been handled by a different element than the desired one.

Android seems to do this by default. This way, if you select a table cell it will read the content of that cell.

* Fix crash in iOS accessbility

Previously, whenever an action would remove the focused element from the screen the app would crash. This change tells iOS to focus the first on-screen element after every change to the semantics tree. This avoids the crash.

In a future iteration, we should tell iOS which element it needs to focus by looking at the sematnics tree (instead of leaving it up to the iOS).

* Better handle the case were the focused element stays on the screen

* review comments

Scrollable elements can now be scrolled in accessibility mode and they scroll in the right direction.

Note: App crashes right after scroll finishes (that's a separate problem as the app currently crashes after every action performed in accessibility mode).

* AX fixes for iOS

With this, the accessibility frame (for VoiceOver, etc.) is drawn at the right position around a widget.

* review comments

The Google style guide specifies a limit of 100 columns.
https://google.github.io/styleguide/objcguide.xml?showone=Line_Length#Line_Length

The Chromium style guide specifies 80.
https://chromium.googlesource.com/chromium/src/+/master/styleguide/objective-c/objective-c.md

* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.

Previously we enabled accessibility unconditionally on iOS, which is more
expensive than necessary. We still enable it unconditionally on the simulator
because there's no API for determining whether accessibility is needed on the
simulator.

Fixes #4849

These classes now use the same terminology and work in the same way.
Also, change semantics.mojom to use an enumeration of actions instead of
having a separate method per action. This will hopefully scale better.

* Wire up accessibility scrolling

flutter/flutter#1664

* Update to mojo 9a791294e429f9f2ca4f427796a284edb6f6768e

* Update to account for Mojo API changes

flutter/flutter#1664

* Move node implementation to Objective C

This changes AccessibilityBridge::Node (C++) to be AccessibilityNode (objc)
in order to have the nodes returned directly as ui accessibility elements.
This buys us a few things:

1) We can now return an element tree instead of a flat list, thus negating
   the need to maintain a global rect and global translation
2) We use less memory since we don't need to instantiate both nodes and
   their ensuing accessibility elements
3) accessibility elements can now implement the methods necessary to respond
   to events like user scrolling

https://github.com/flutter/flutter/issues/1664

All the headers for Flutter.framework are now in framework/Headers and all the
source files for Flutter.framework are now in framework/Source. Previously it
was unclear that FlutterAppDelegate and main_ios.mm weren't part of
Flutter.framework. (They still build as part of the framework, but that's
something I'll fix in a later patch.)

Report Flutter widgets to iOS accessibility API