Returns the controller stored by SetController.
The controller for this control.
If no controller is set, a reference to the control itself is returned.
The default implementation uses a field to retrieve the reference.
If external controller is not specified, control will serve as its own controller, which will allow data binding to work properly.
You may override this method e.g. to return Controller in order to have your control bind to the same controller as your page. When overriding this behaviour, derived classes must ensure to also change the behaviour of SetController accordingly.