Requiring the compositor private object to be a GObject allows the
return value of meta_window_get_compositor_private() to be used
via gobject-introspection, since the type and memory management
can be determined.

This allows a reverse mapping from MetaWindow to MutterWindow.

- Change meta_window_get/set_compositor_private to use GObject
  rather than void *
- Adapt code in compositor-mutter.c to match.