Move Clutter backend code to src/backends
There's a few bits and pieces of Clutter that are still backend-y:
- ClutterStageCogl and ClutterStageViewCogl are the base of all backend stage/view implementations
- ClutterBackendX11 has most of the X11 Clutter backedn implementation
Move this code into src/backends, adding private MetaStageImpl and MetaStageView base objects there, and making MetaClutterBackendX11 take over ClutterBackendX11 functionality.
This finally removes all traces of backend code from the Clutter tree, making it all live in src/backends.