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