Add didComplete parameter to the completion blocks

[jverkoey]

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Adding support for explicit animations is trivial because we can do a one-to-one mapping of animations to completion handlers.

Adding support for implicit animations, however, is non-trivial because multiple animations map to the same completion handler. We'll have to implement the animation delegate on each added animation and coalesce the overall completion state into a single invocation of the completion handler.

This would bring our API closer in alignment to UIView's animation APIs.