window.go
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}
	}
}
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}
	}
}
// OnKeyDown sets the callback that is called when a user presses a key
// while the Window is active