code: func (w *Window) OnKeyDown(fn func(Event, interface{}) bool, data interface{}) {
  	if fn == nil {
  		w.onKeyDown = nil
  	} else {
  		w.onKeyDown = &keyDownCb{data: data, fn: fn}
  	}
  }
docs: // OnKeyDown sets the callback that is called when a user presses a key
  // while the Window is active