libgimpwidgets, app, plug-ins: fix last GIR warnings.
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.
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