code: func (d *DropDown) SetSelectedFunc(handler func(text string, index int)) *DropDown {
  	d.selected = handler
  	return d
  }
docs: // SetSelectedFunc sets a handler which is called when the user changes the
  // drop-down's option. This handler will be called in addition and prior to
  // an option's optional individual handler. The handler is provided with the
  // selected option's text and index. If "no option" was selected, these values
  // are an empty string and -1.