AnimationCollection usability improvements

[pjcozzi]

In all fairness, AnimationCollection (Sandcastle example) is not complete, but people are using it and it has the potential to bring a lot of coolness to apps. I wrote it in a hurry way before we went open-source.

It needs some re-envisioning and flushing out, but here are a few specific issues from the transit hackathon:

• An animation needs to be robust if the parent object it is animating is destroyed, e.g., if isDestroyed is true, the animation should remove itself. This is too much for the app to handle.
• Setting easingFunction for addProperty is too hard. The options are not documented and require reaching into third-party types, e.g., easingFunction : Cesium.Tween.Easing.Cubic.In.
• Implementing non-trivial things like animating a billboard on mouse over requires way too much state management. Higher-level constructs are needed.

[mramato]

CC #377 (We no longer have the two widgets example, but this is still a problem in the overall animation architecture).

[mramato]

@pjcozzi can we close this, or should it just have the 1.0 tag removed after your above PRs?

[pjcozzi]

All good to close. Anything additional is covered in #1830.