GtkCssStyle is the base class to be used for all types of styles that do
exist.
GtkCssAnimatedStyle is the only implementation so far, that is exactly a
copy/paste of the old GtkCssStyle code.
This will be useful to not trigger updates all the time when nothing is
happening (ie due to animations being paused or due to them having
reached their final value).
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.
