stage: Add state tracking

stage: Add state tracking

State changes on the Stage are currently deferred to the windowing
system backends, but the code is generally the same, and it should
be abstracted neatly inside the Stage class itself.

There's also the extra caveat for backends that state changes on a
Stage must also emit a ClutterEvent of type CLUTTER_STAGE_STATE, a
requirement that needlessly complicates the backend code.