Optional _delegate in charge of Window-related events. Provides nice isolation for unit testing, and allows customization of confirmation handling.

abstract class Delegate {
 /**
  * Adds a {@link ClosingHandler} to the Delegate.
  * 
  * @param handler a {@link ClosingHandler} instance
  * @return a {@link HandlerRegistration} instance
  */
 HandlerRegistration addWindowClosingHandler(ClosingHandler handler);

 /**
  * Called to confirm a window closing event.
  * 
  * @param message a warning message
  * @return true to allow the window closing
  */
 bool confirm(String message);
}