Responder chain

[jdolan]

This pull request completely changes our draw and event handling algorithms:

• Views are no longer "accumulated" and then sorted on the Renderer
• Instead, they are drawn in-order using depth-first recursion
• zIndex is completely gone: Views are drawn in natural order.
• Use natural ordering to arrange Views from back to front.
• A hack exists to defer rendering of the firstResponder so it is always on top
• Event dispatch now uses a responder chain that uses "bottom-up" recursion
• Only the Views which directly receive Events (by x,y coordinate) are eligible to respondToEvent
• Control classes are simplified, because they no longer need to check if they received events
• Panel is now a Control and assumes firstResponder accordingly
• Views now maintain a reference to their Window via a recursive setter
• No more calls to SDL_GL_GetCurrentWindow peppered throughout View
• New Notification object for broadcasting messages to all ViewControllers
• Constraints can now self-reference, for e.g. preserving aspect ratios
• New ViewController lifecycle methods (viewDidAppear, viewDidDisappear)

[jdolan]

TODO:

• [ ] Tie ViewControllers into responder chain elegantly (introduce Responder class?)
• [ ] First responder not drawn late enough, can still have draw order issues
• [x] renderDeviceDidReset not working?