code: public static MessageBox showYesNoMessageBox(String title, String question, Callback<Boolean, Void> onFinished) {
  		MessageBox box = new MessageBox(title, question, onFinished);
  		box.setMessageStyleType(MessageStyleType.HELP);
  		box.setMessageBoxType(MessageBoxType.YESNO);
  		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.
  <p>
  if the dialogbox is closed with the closebutton instead of a button, onFinished.onFailure will be called.
  
  @param title
  @param question
  @param onFinished
  @return