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].
Instead of bending GtkGizmo to the breaking point,
split off a GtkPopoverContent class that just does
what is needed here. Its simpler, and lets us keep
GtkGizmo simpler too.
