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
The colorbutton contains a button which contains a colorswatch.
We want the focus to go straight to the button, nowhere else,
so mark the swatch as !can-focus.
Adapt tests to match.
