Use "later functions" to fix priority problems with Clutter redraw

There was a problem where if, for example, a restack was triggered
out of a clutter event handler, then after Clutter processed the
events, it would proceed immmediately on to repaint the stage without
ever returning control to the GLib main loop. So even though we
had an idle handler installed with a higher priority than the
Clutter stage repainting the clutter stage repainting would happen
first and we'd get a wrong frame.

Fix this by introducing the idea of "later functions", which abstract
the idea of "doing something later" away from g_idle_add() and use
a combination of GLib idle functions and Clutter "repaint functions"
to get our callbacks triggered at the right time, even when they
are installed from a clutter event handler.

https://bugzilla.gnome.org/show_bug.cgi?id=596334

This also resolve a FIXME where MUTTER_PRIORITY_BEFORE_REDRAW
could starve stage repainting.