This patch makes the FlutterViewController interface more idiomatic by not
retaining listeners. It's the callers responsibility to make sure the lifetimes
work out.

This patch switches us to building Runner.app on the end-developer's machine,
which will eventually let them customize their app.

Fixes https://github.com/flutter/flutter/issues/3256

After this patch, main_ios.mm now consumes Flutter.framework via public APIs.
This patch prepares us to move Flutter on iOS over to consuming
Flutter.framework as a binary artifact.

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.)

Also cleans up the messages when errors occur in both modes. I have also made the API a lot simpler and moved more of the extraneous stuff out of public. Only FlutterViewController and the Dart project configuration are publically available.

* Remove engine code from SkySurface into the controller.
* The public API exposed by Flutter.framework for third-party embedders is present ios/public.
* Public view controller API uses idiomatic Objective-C and is subclassable by consumers.
* Paves way for moving the embedder into a framework.