stylecontext: Add an animating framework

stylecontext: Add an animating framework

The design principles were:

- synchronized
If multiple style contexts are animating, they should all do an
animation step at the same time.

- degrades well
Even when there's thousands of style contexts all animating at the same
time, the animation steps don't starve the CPU. This is achieved by
making sure the timeout is really fast. It just sets a bunch of flags.

- no hidden bottlenecks
Turning animatability on or off on a style context is O(1).

So far it is unused.