filechooser: Disconnect signal handlers from search engine before destroying

The search engine might stay alive longer due to extra temporary refs, so
the signal handlers should be removed for the filechooser to ignore these
properly.

https://bugzilla.gnome.org/show_bug.cgi?id=745479

gtk/gtkfilechooserwidget.c

@@ -6205,7 +6205,7 @@ search_stop_searching (GtkFileChooserWidget *impl,
   if (priv->search_engine)
     {
       _gtk_search_engine_stop (priv->search_engine);
-      
+      g_signal_handlers_disconnect_by_data (priv->search_engine, impl);
       g_object_unref (priv->search_engine);
       priv->search_engine = NULL;
     }