xwayland: Use single frame callback for Present flips and normal updates

Using a list of Present windows that need to be called back.

This prepares for the following change, there should be no change in
observed behaviour.

v2:
* Use xwl_window_create_frame_callback instead of making the
  frame_listener struct non-static (Olivier Fourdan)

Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>