code: public static MessageBox showYesNoCancelMessageBox(String title, String question, Callback<Boolean,
  			Void> onFinished) {
  		MessageBox box = new MessageBox(title, question, onFinished);
  		box.setMessageStyleType(MessageStyleType.HELP);
  		box.setMessageBoxType(MessageBoxType.YESNOCANCEL);
  		box.center();
  		return box;
  	}
docs: This will show a MessageBox expecting an answer.
  <p>
  onFinished.onSuccess will be called with true for yes, and false for no. onFinished.onFailure will be called for
  cancel and when the dialogbox is closed with the closebutton.
  
  @param title
  @param question
  @param onFinished
  @return