Remove unnecessary copy constructor from WritableVirtualController.
Introduce the new input functionality. Some old buttons are now obsolete.
Migrate all controller clients to use the new buttons.
Add comments for the new virtual buttons.
Modify key mappings to fit the new input spec.
Input handlers should be registered only when the screen has loaded all of its content.
Add base.LoadContent call to BackgroundScreen.
Fixes #99
Begin using a reactive approach to input, starting with the InputComponent and adapters.
Integrate reactive input into GameScreen.
Make GameplayScreen work well with reactive input.
Fix annoying reactive input bug missing the first keydown.
Merge branch 'input-spec'

Conflicts:
	PuzzlePathDimension/Menus/GameEditorScreen.cs
	PuzzlePathDimension/Menus/GameScreen.cs
	PuzzlePathDimension/Menus/ToolboxScreen.cs
	PuzzlePathDimension/Models/TopLevelModel.cs