* clutter/clutter-container.[ch]: Add a generic actor container
interface, for actors to implement without subclassing ClutterGroup.
* clutter/clutter-group.[ch]: Make ClutterGroup implement the
ClutterContainer interface, and deprecate the colliding methods.
* clutter/clutter-layout.[ch]: Add extended layout interface.
Actors and containers requiring or honouring complex layout
management should implement this interface and provide at least
one of the available layout types: width for height, height for
width, natural size, iterative size request.
* clutter/clutter-label.c: A ClutterLabel requires height for
width layout management, so it implements the ClutterLayout
interface.
* clutter/Makefile.am: Add new files to the build.
* tests/*.c:
* examples/*.c: Update tests and examples code to use the
new ClutterContainer API instead of ClutterGroup.
