* clutter/clutter-actor.c:
* clutter/clutter-actor.h:
Clean up of parenting code (see #972)
Doc updates to section intro.
Add clutter_actor_allocate_preferred_size () utility call
* clutter/clutter-group.c: (clutter_fixed_layout_allocate):
Use clutter_actor_allocate_preferred_size ()
* doc/clutter-actor-invariants.txt:
Add some more notes