Drop the custom style printing implementation in gtkcssnode.c and
instead reuse the existing gtk_css_style_print function, extending
it a bit to suit our needs.
Instead of computing values, just recognize initial values by
having no CSS section. Also do away with the show-initial flag, and
just always filter out initial values. The flag can come back when
it is needed.
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.
