* README:
        Add notes on new multistage feature.

        * clutter/clutter-stage-manager.c:
        Dont ref contained stages.

        * clutter/clutter-stage.c:
        Automatically remove stage from stage manager on finalisation.
        Cleans up warnings when a stage is destroyed.

        * clutter/clutter-backend.h:
        * clutter/glx/clutter-backend-glx.c:
        Minor formatting cleanups.

        * clutter/glx/clutter-stage-glx.c:
        * configure.ac:
        * clutter/clutter-version.h.in:
        Add a general CLUTTER_STAGE_TYPE define, should be useful for
        evntual stage subclassing and creating with g_object_new()