gimp_int_radio_group_new() was still complaining about the scope of
radio_button_callback(). Make it (scope notified) because it needs to
stay alive after the function returns and may be called multiple times.

Also adding a GDestroyNotify to free the callback data once the widget
is destroyed (additionally it will also serve as a notifier for bindings
to properly free the callback closure itself, not only it's data).

With this last one done, GObject Introspection generation now happens
without any warning output.