message: qt, refactor: Keep HelpMessageDialog in the main event loop
diff: @@ -940,8 +940,8 @@ void BitcoinGUI::aboutClicked()
  if(!clientModel)
  return;
  - HelpMessageDialog dlg(this, true);
  - dlg.exec();
  + auto dlg = new HelpMessageDialog(this, /* about */ true);
  + GUIUtil::ShowModalDialogAndDeleteOnClose(dlg);
  }
  void BitcoinGUI::showDebugWindow()
  