gdbus-codegen: Support extensions and changing the GDBusInterfaceSkeleton type to inherit from

Same idea as !4805 (closed), but only changes the things to make it possible to implement the required codegen and GDBus subclassing in libdex itself.

The corresponding libdex MR: libdex!28

The updated test repo: https://gitlab.gnome.org/swick/gdbus-libdex-codegen-test