Plug a memory leak in clutter-table-layout

Plug a memory leak in clutter-table-layout

Whenever the allocation is changed on a child of a ClutterTableLayout
and animations are not in effect then it would store a copy of the
allocation in the child meta data. However it was not freeing the old
copy of the allocation so it would end up with a small leak.

Instead of just changing it to free the old value this patch makes it
store the allocation inline in the meta data struct because it seems
that the size of an actor box is already quite small compared to the
size of the meta data struct so it is probably not worth having a
separate allocation for it. To detect the case when there has not yet
been an allocation a separate boolean is used instead of storing NULL.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2358