clutter-backend-win32: Use g_object_unref on the stage manager

clutter-backend-win32: Use g_object_unref on the stage manager

Previously when trying to destroy all of the stages in the backend
dispose function it would poke directly in the ClutterStageManager
struct to get the list. In 8613013ab08 the defintion of
ClutterStageManager moved to a different header which isn't included
by the Win32 backend so it wouldn't compile. In that commit the X11
backend was changed to unref the stage manager instead of poking in
the internals so we should do the same for the win32 backend.