wayland: Emit signal on primary selection data source destruction

This is necessary for the X11 side to catch up, and unset the primary
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

@@ -1515,6 +1515,8 @@ primary_source_destroyed (gpointer  data,
       if (data_device_resource)
         gtk_primary_selection_device_send_selection (data_device_resource, NULL);
     }
+
+  wl_signal_emit (&data_device->primary_ownership_signal, NULL);
 }
 
 void