Aura WM and Shell

To allow us to perform large scale window transitions, we need to back Windows by compositor layers so that we can animate them without redrawing.

The aura::Window wraps a Compositor Layer. It also has a delegate which responds to events and paints to its layer.

aura::Windows are similar to Views, only simpler, they are a hierarchy that live within an aura::Desktop, which is where the real platform-specific code lives.