A radio button is a special kind of checkbox that is used to select exactly
one option from a set of options. Radio buttons belong to a RadioGroup that
holds all related radio buttons from which only one option can be selected.

See also:
Constructor
new(group:RadioGroup, label:String)
Creates a new RadioButton.
Parameters:
| group | The  | 
|---|---|
| label | The label of this radio button. |