Instead of using ClutterActor for the base class of the Stage
implementation we should extend the StageWindow interface with
the required bits (geometry, realization) and use a simple object
class.

This require a wee bit of changes across Backend, Stage and
StageWindow, even though it's mostly re-shuffling.

First of all, StageWindow should get new virtual functions:

  * geometry:
    - resize()
    - get_geometry()

  * realization
    - realize()
    - unrealize()

This covers all the bits that we use from ClutterActor currently
inside the stage implementations.

The ClutterBackend::create_stage() virtual function should create
a StageWindow, and not an Actor (it should always have been; the
fact that it returned an Actor was a leak of the black magic going
on underneath). Since we never guaranteed ABI compatibility for
the Backend class, this is not a problem.

Internally to ClutterStage we can finally drop the shenanigans of
setting/unsetting actor flags on the implementation: if the realization
succeeds, for instance, we set the REALIZED flag on the Stage and
we're done.

As an initial proof of concept, the X11 and GLX stage implementations
have been ported to the New World Order(tm) and show no regressions.