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.