New display architecture

[glen.low@…]

We're revamping the display architecture around multiple animation layers rather than one graphics context. This should give us the following improvements:

• Improved graph display during scrolling and zooming. Graph should display at full resolution (except for text) during scrolling instead of displaying a checkerboard pattern. Zooming should be cleaner and more reliable.
• Graph changes will now animate. Created objects will fade in, destroyed objects will fade out, changed objects will morph across. This will better cue user changes.
• Lay the foundation for future work. In particular, it will be easier to selectively hide, display or otherwise modify a single component display, rather than having to rerender everything.

[glenlow]

(In [160]) Improved graph display during scrolling and zooming; graph changes will now animate (fix #97). New display architecture is layer-based rather than context-based. Refactored GraphKitDraw? classes into core classes with properties and categories that implement the actual drawing. Component destruction now notifies observers before destruction, helping with component housekeeping in GKDrawingBoard and IVDrawAdditions.