A RadioToolButton is a ToolItem that contains a radio button, that is, a button that is part of a group of toggle buttons where only one button can be active at a time.
Use RadioToolButton to create a new GtkRadioToolButton. Use RadioToolButton.from_widget to create a new GtkRadioToolButton that is part of the same group as an existing GtkRadioToolButton.
GtkRadioToolButton has a single CSS node with name toolbutton.