* Create FlutterEngine to manage a Shell to support maintaining execution state across instances of FlutterViewControllers
* Refactor PlatformViewIOS to support adding or removing a FlutterViewController dynamically
* Refactor FlutterDartHeadlessCodeRunner to implement FlutterEngine
* Refactor FlutterViewController to accept a FlutterEngine at initialization or to create one for backwards compatibility; any Shell related responsibilities are now passed through to the Engine instance
* Remove FlutterNavigationController (unused)
* Update all public Objective C doc comments to be consistent and formatable
* Add public documentation to all public headers

For flow to manipulate the embedded UIViews during the paint traversal
it needs some hook in PaintContext.
This PR introduces a ViewEmbeder interface that is implemented by the
iOS PlatformViewsController and plumbs it into PaintContext.

The ViewEmbedder interface is mainly a place holder at this point, as
this PR is focused on just the plumbing.

* Update FlutterPlugin.h docs, suppress warning for older API

On Windows the ERROR macro is defined by some headers which breaks the
FML_LOG(ERROR).

https://skia.googlesource.com/skia.git/+log/3b79aa3a5ad0..b53f1f46982d


Created with:
  gclient setdep -r src/third_party/skia@b53f1f46982d

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