libxfce4ui

[Nick Schermer]

Started with some basic functions. I'm not really sure
xfce_gdk_{display,screen}_get_fullname belong in the library since
they are only used in xfce4-session (and I can hardly imagine some
other apps need them). So maybe it's better to move them.

Also removed xfce-heading from the public api for 2 reasons: it's not
used by any application at the moment; it's easier to add interfaces
then remove them in the feature.

Also added a XfceFrame, because it's more flexible then the old
solution and the amout of code is minimal.

http://foo-projects.org/~nick/tmp/libxfce4ui/

Nick