Since ClutterActor directly calls ClutterGroup methods we need ClutterContainer
to provide them, so that every container actor behaves the same way.
This patch makes ::raise, ::lower and ::sort_depth_order virtual functions
of the ClutterContainer interface, implemented by ClutterGroup.
The documentation has been added and deprecation warnings have been added
as well.
Finding a child by its ID is a method that should be used by every container
and not only by ClutterGroup. This is needed to actually fix the picking in
ClutterStage.
