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.
Every member of the ButtonGroup should have BaseButton.toggle_mode set to
get_buttons ( )
pressed ( BaseButton button )
Emitted when one of the buttons of the group is pressed.
BaseButton get_buttons ( )
BaseButton get_pressed_button ( )
Returns the current pressed button.