GUI Editor: export JS module with control accessors

[jelster]

Thing through this some more, here are some refinements:

1. ES6 module format!

2. Module should expose an async initialize API that internally loads the GUI up and preps it for use

3. Exposing update methods as observers - perhaps we generate Observer functions that can subscribe to Observables producing updated values for the control. If set accessors are suited that would work better. Example: onScoreChangedObservable.add(guiModule.score.onUpdate)

4. (Optional) allow or provide pre-set AnimationGroups and/or Animations for actions like fade in/out, etc.

5. (Optional) expose control life cycle events like onResize as further sets of Observables