ClutterGroup @cluttergroup: the object which received the signal. @arg1: @cluttergroup: the object which received the signal. @arg1: @Returns: @self: @Returns: @self: @callback: @user_data: @self: @actor: @self: @first_actor: @args: @self: @first_actor: @Varargs: @self: @actor: @self: @self: @self: @id: @Returns: @self: @actor: @sibling: @self: @actor: @sibling: @self: