In particular, make it so that we call meta_compositor_add_window() on
a new window before calling meta_compositor_sync_stack() to position
it. The list returned by mutter_plugin_get_windows() is only updated
by sync_stack(), but sync_stack() only pays attention to windows that
add_window() has already been called on. So without this change, a
newly-mapped window will not be returned by
mutter_plugin_get_windows() until after the *next* restacking.
http://bugzilla.openedhand.com/show_bug.cgi?id=1512
f78390b094
