contained data structure.
* clutter/clutter-id-pool.[ch]: new files.
* clutter/Makefile.am: added clutter-id-pool.[ch]
* clutter/clutter-private.h: use a ClutterIDPool instead of GArray and
GSList to keep track of the reusable ids.
* clutter/clutter-actor.c: moved id pool logic away.
* clutter/clutter-main.c: simplified id pool creation/finalization.
