container: Expose create and destroy ChildMeta methods

container: Expose create and destroy ChildMeta methods

It is conceivable that Container implementations might add children
outside of the Container::add() implementation - e.g. for packing at
a specific index. Since the addition (and removal) might happen outside
the common path we need to expose all the API that is implicitly called
by ClutterContainer when adding and removing a child - namely the
ChildMeta creation and destruction.