Session client support in libxfce4ui

[Nick Schermer]

Well the session management support in exo looks good enough to me for
normal applications. Does this still work with the latest x.org
releases Benny? And can save-yourself be used to save and quit, like
macosx does (kAEQuitApplication or applicationShouldTerminate)?

We could then use something like Brian's desktop files for starting
applications.

Nick