Group of Buttons. Group of [BaseButton]. The members of this group are treated like radio buttons in the sense that only one button can be pressed at the same time. Some types of buttons (such as [CheckBox]) may have a special appearance for this state. Every member of the ButtonGroup should have [member BaseButton.toggle_mode] set to [code]true[/code]. Returns an [Array] of [Button]s who have this as their [ButtonGroup] (see [member BaseButton.button_group]). Returns the current pressed button. Emitted when one of the buttons of the group is pressed.