Bug #864 - Allow instantiating and subclassing of ClutterStage

	* clutter/Makefile.am: Add clutter-stage-window.[ch]

	* clutter/clutter-stage-manager.c:
	(_clutter_stage_manager_remove_stage): Do not warn if removing
	a stage we don't manage, as we might be invoked multiple times
	during a ClutterState dispose sequence.

	* clutter/clutter-actor.c:
	* clutter/clutter-backend.[ch]:
	* clutter/clutter-main.c:
	* clutter/clutter-private.h:
	* clutter/clutter-stage.[ch]: Make ClutterStage a proxy actor,
	with a private actor implementing the ClutterStageWindow
	interface for handling the per-backend realization, painting
	and unrealization, plus all the windowing system abstraction.

	* clutter/x11/clutter-event-x11.c:
	* clutter/x11/clutter-stage-x11.[ch]: Port the X11 backend
	to the new backend and stage API and semantics.

	* clutter/glx/clutter-backend-glx.c:
	* clutter/glx/clutter-stage-glx.c: Port the GLX backend to
	the new backend and stage API and semantics.

	* clutter/eglx/clutter-backend-egl.[ch]:
	* clutter/eglx/clutter-stage-egl.[ch]: Port the EGLX backend
	to the new backend and stage API and semantics (untested).

	* tests/test-multistage.c (on_button_press): Rename
	clutter_stage_create_new() to clutter_stage_new().