code: func (ww *WidgetWatchers) Watch(handler tcell.EventHandler) {
  	if ww.watchers == nil {
  		ww.watchers = make(map[tcell.EventHandler]struct{})
  	}
  	ww.watchers[handler] = struct{}{}
  }
docs: // Watch monitors this WidgetWatcher, causing the handler to be fired
  // with EventWidget as they are occur on the watched Widget.