A radiobutton without indicator is really just a togglebutton with a
group.
A radiobutton with indicator is really just a checkbutton with a group.
Make checkbutton its own widget not inheriting from GtkButton.
GtkRadioButton could be removed but it stays for now.
Radiobutton && !draw-indicator => Togglebutton
Checkbutton && !draw-indicator => Togglebutton
Radiobutton && draw-indicator => CheckButton + group
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