DrawEngine as a GameObject.

[JSideris]

A pretty radical idea.

• [x] Ticker will go back to being a requestAnimationFrame in the GameClientPlugin which will send out renderUpdate events.
• [x] No more abstract DrawEngine. Move everything into DrawEngine2d or elsewhere.
• [x] Refactor DrawEngine2d to represent the PIXI world. The canvas plugin can still have a reference to it.
• [x] Refactor DrawEngine2d's lifecycle around GameObject's.
• [x] The new Painter2d can let the current 2D DE know about the creation of objects that it extends.