Instead of keeping an animated style everywhere, we replace it with the
static style when nothing gets animated.
Apart from making the code cleaner, this gets rid of a bunch of animated
style values that do nothing but wrap a static style.
For now, this is only an implementation detail of the animated style.
The idea is to use GtkCssStaticStyle as the result of CSS queries and
then put a GtkCssAnimatedStyle on top that manages the animations. The
neat thing about this is that you can cache the static values.
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.
