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 seems a bit "too powerful" and unlikely to be used by most
applications. Remove it from now, until someone comes up with a strong
desire for it.
https://bugzilla.gnome.org/show_bug.cgi?id=670485
We don't expose ::quit-requested as API anymore. Instead, we expect
users to register inhibitors when needed. Without quit-requested,
there is no need for ::quit-cancelled and gtk_application_quit_response
anymore.
We still emit ::quit when the application is about to quit.
