xwayland: Make window_get_none_wm_owner return a Window instead of a Client

Make window_get_none_wm_owner return the first non-wm-window instead of the
owner (client) of the first non-wm-window and rename it to
window_get_client_toplevel to match its new behavior.

This is a preparation patch for switching to using the drawable coordinates
in xwl_window_should_enable_viewport()

Changes by Hans de Goede:
- Split this change out into a separate patch for easier reviewing
- Rename window_get_none_wm_owner to window_get_client_toplevel to match
  its new behavior

Signed-off-by: Roman Gilg <subdiff@gmail.com>
Acked-by: Olivier Fourdan <ofourdan@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>