Add support for additive Core Animation tweens.
Additive animations are the preferred mechanism for building cancelable tweens. Any tween type that implements the Subtractable protocol will now be additive by default.
Additive animations are also the default animation type for UIView implicit animations as of iOS 8 or 9 or 10 .
Reviewed By: O4 Material Apple platform reviewers, chuga
Tags: Material Motion
Differential Revision: http://codereview.cc/D2891