css: Split styles into groups

css: Split styles into groups

Introduce refcounted structs for groups of related css properties,
and use them to store the style values. Both GtkCssStaticStyle and
GtkCssAnimatedStyle fill in the structs in GtkCssStyle, and we
can avoid vfuncs for value access, which should be much faster.
We can even start accessing style->core->color directly.