Menu module in GNOME Panel

For very long time I am working on new libgnome-panel library. It is still private library, but at some point it will become public replacing old libpanel-applet library. My own requirement to make it public is that all internal objects are ported to new library.

After GNOME Panel 3.30 release I have finished converting menu objects to menu module. To keep existing functionality I had to introduce new functionality that will be available also to 3rd party applet developers once library will be public.