I believe you might want to include a .desktop file (installing it in
e.g. /usr/share/applications/), and it should be used by all compatible
DE-s.
You can use whatever files you have there as examples, and the latest
specs should be these:
http://standards.freedesktop.org/desktop-entry-spec/late
This is not strictly speaking a gtk+ question, but I was hoping someone
might help me out anyway: how do I go about integrating my application
properly with the desktop, i.e., so that my program shows up with an
entry and an icon in the application menu of Gnome, Xfce4, and all other
compatible des