There is a widespread need to access the CSS foreground
color for custom drawing in snapshot functions, so make
it available after gtk_style_context_get_color was
deprecated with a new widget api.
The notable exception here are the global provider apis,
which are needed in some form and don't have a replacement
yet. Move them to gtkstyleprovider.[hc], so we can wholly
deprecated gtkstylecontext.[hc].
