popdown the GtkMenu before detach GtkMenu

If we detach a GtkMenu while it still being displayed, it would freezes entire panel as described in issue lxde#83, so we popdown the GtkMenu to avoid Freeze bug

plugins/task-button.c

@@ -1544,6 +1544,7 @@ void task_button_update_windows_list(TaskButton *button, Window *list, gint n)
         for (l = menu_list; l; l = l->next)
         {
             GtkMenu *menu = GTK_MENU(l->data);
+
             gtk_menu_popdown(menu);
             gtk_menu_detach(menu);
         }