Adds this to the group of
In a group of multiple check buttons, only one button can be active at a time.
Setting the group of a check button also changes the css name of the indicator widget's CSS node to 'radio'.
The behavior of a checkbutton in a group is also commonly known as a 'radio button'.
Note that the same effect can be achieved via the Actionable api, by using the same action with parameter type and state type 's' for all buttons in the group, and giving each button its own target value.
another CheckButton to form a group with