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.
Otherwise, that value will never get reset and remain frozen in time.
This is problematic for example when the value is inherited and the
parent changes the value.
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).
This is to allow animating arrays properly. I'm not really thrilled
about this solution (we leak propertys into the values again...), but
it's the best I can come up with - I prefer it to having N different
array types...
