Change button hierarchy

Change button hierarchy

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