Redo CSS style printing

Redo CSS style printing

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.