Store a back pointer of the layout manager inside the container using
the GObject instance data. This introduces a change in the implementation
of ClutterLayoutManager, though it's still binary compatible.
When set_container() is called with a NULL container we cannot use the
passed pointer to unset the CLUTTER_ACTOR_NO_LAYOUT flag. We should
store a back pointer to the container as object data (there's no need
to add a Private data structure in this case) and unset the flag on the
back pointer instead.
Previously only ClutterGroup was able to set the CLUTTER_ACTOR_NO_LAYOUT
flag which allows clutter-actor.c to avoid a relayout when showing or
hiding fixed layout containers. Instead of it being the responsibility
of the container to set this flag this patch makes the layout manager
itself decide in the ::set_container method. This way both ClutterBox
and ClutterGroup can take advantage of the optimization.
