stylecontext: Add new class for animation handling

stylecontext: Add new class for animation handling

A StyleAnimation is an immutable object used to track the state of CSS
values. I'd have liked to make it fully immutable - ie not have the
timestamp in there - but couldn't find a place to sanely store the
timestamp.

This is an abstract base class. Implementations for this will be added
later (for both CSS3 transitions and animations, potentially for
animated images).

Actually aplying the information in this object will be done by a
different object commtted later.