Remove st_container_remove_all & rewrite st_container_destroy_children

1. Both functions leaked the nodes in priv->children
2. st_container_remove_all wasn't properly updating first_child and last_child
3. remove_all() is almost never right since it won't cause signal handlers
   on the children to be removed. In the rare cases where it might be needed
   the caller can simply use clutter_container_remove().
https://bugzilla.gnome.org/show_bug.cgi?id=640781

js/ui/dash.js

@@ -206,7 +206,7 @@ Dash.prototype = {
 
     _redisplay: function () {
         this._box.hide();
-        this._box.remove_all();
+        this._box.destroy_children();
 
         let favorites = AppFavorites.getAppFavorites().getFavoriteMap();