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
This test is a bit brittle because it doesn't properly rely on CSS
properties but needs to use widget style properties to turn of extra
sizing from widgets.
It might break in the future when porting widgets to draw properly.
