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