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/modalDialog.js

@@ -83,7 +83,7 @@ ModalDialog.prototype = {
     },
 
     setButtons: function(buttons) {
-        this._buttonLayout.remove_all();
+        this._buttonLayout.destroy_children();
         let i = 0;
         for (let index in buttons) {
             let buttonInfo = buttons[index];