Add conveniences for animating collection properties

[NickEntin]

Currently there isn't a great way to animate a collection of subelements.

For example, say your element has a property containing an array of subviews that should each be animated. If the count is known during construction, you can add each one by key path, but you lose the compiler safety by using indices in the key path. The other option here is to use an AnimationGroup, which brings back the compiler safety but loses the separation of construction and execution.

In order to support building animations of collections for which the number of elements isn't known during construction, we need a new type of content that can be added to the Animation.

[NickEntin]

There's a work-in-progress implementation of this in the framework currently, under the name Collection Keyframes. This enables adding keyframes that will be applied to all elements in a collection. The timestamp for each keyframe can be determined based on the element's index in the collection.

Collection Keyframes are marked as internal since there's still a lot of work left to do, especially around how they interact with other animation content (like how they behave in nested animations).

[NickEntin]

The work-in-progress implementation was removed in #46.

[NickEntin]

In the year that this issue has been open, I've haven't heard about this being a pain point at all. I'm going to close it out as a won't-fix for now. If anyone has a use case where this functionality would make a big difference, please comment - it'd be great to hear how this could be improved!