wayland: Emit signal on selection data source destruction

This is necessary for the X11 side to catch up, and unset the selection
ownership on our window that represents the wayland side in X11 selection.

https://bugzilla.gnome.org/show_bug.cgi?id=760745

src/wayland/meta-wayland-data-device.c

@@ -1117,6 +1117,8 @@ selection_data_source_destroyed (gpointer data, GObject *object_was_here)
       if (data_device_resource)
         wl_data_device_send_selection (data_device_resource, NULL);
     }
+
+  wl_signal_emit (&data_device->selection_ownership_signal, NULL);
 }
 
 static void