gio ActionMap/ActionGroup subclassing support

[bilelmoussaoui]

It's pretty useful to be able to subclass gio::ActionMap when creating a custom widget with it's own actions group instead of having to add an actions field into your imp::WidgetStruct

[sdroege]

See https://github.com/gtk-rs/gobject-subclass/tree/master/gio-subclass for the very old version of those. They need some special care because of all the transfer none return values.