iOS Add2App Test

This application demonstrates some basic functionality for Add2App, along with a native iOS ViewController as a baseline and to demonstrate interaction.

The following functionality is currently implemented:

1. A regular iOS view controller (UIViewController), similar to the default flutter create template.
2. A FlutterViewController subclass that takes over full screen. Demos showing this both from a cold/fresh engine state and a warm engine state.
3. A demo of pushing a FlutterViewController on as a child view.
4. A demo of showing both the native and the Flutter views using a platform channel to to interact with each other.
5. A demo of showing two FlutterViewControllers simultaneously.

A few key things are tested here:

1. The ability to pre-warm the engine and attach/detatch a ViewController from it.
2. The ability to simultaneously run two instances of the engine.
3. The ability to use platform channels to communicate between views.
4. That a FlutterViewController can be freed when no longer in use.
5. That a FlutterEngine can be freed when no longer in use.