ClutterStage: Replace clutter_stage_set_paint_callback() with ::after-paint signal

clutter_stage_set_paint_callback() has the disadvantage that it only
works for a single caller, and subsequent callers will overwrite and
break previous callers. Replace it with an ::after-paint signal that is
emitted at the same point - after all painting for the stage is
completed but before the drawing is presented to the screen.

https://bugzilla.gnome.org/show_bug.cgi?id=732342