Label (attribute of actiongroup)
The label is the visible name of the actiongroup, and it's not required.
Depending on the widget it will be rendered:
- widget 'select': the label is the 1st option of the SELECT element (if no label, the 1st option is empty)
- widget 'div': the label is the LEGEND of a FIELDSET surrounding the children options (if no label, no fieldset, like in column)