ToggleButton.setGroup

Adds self to the group of group.

In a group of multiple toggle buttons, only one button can be active at a time.

Setting up groups in a cycle leads to undefined behavior.

Note that the same effect can be achieved via the gtk.actionable.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.