code: func (d *DropDown) SetDoneFunc(handler func(key tcell.Key)) *DropDown {
  	d.done = handler
  	return d
  }
docs: // SetDoneFunc sets a handler which is called when the user is done selecting
  // options. The callback function is provided with the key that was pressed,
  // which is one of the following:
  //
  //   - KeyEscape: Abort selection.
  //   - KeyTab: Move to the next field.
  //   - KeyBacktab: Move to the previous field.