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/workspacesView.js

@@ -769,7 +769,7 @@ WorkspaceIndicatorPanel.prototype = {
         else
             this.actor.set_skip_paint(this._box, false);
 
-        this._box.remove_all();
+        this._box.destroy_children();
         for (let i = 0; i < this._workspaces.length; i++) {
             let actor = new St.Button({ style_class: 'workspace-indicator',
                                         track_hover: true });