Ubuntu – What .desktop file options are available

It looks like we've acquired quite a few new options in desktop files lately. Some of these are explained in very narrow posts, such as how to add a static quicklist to a Unity LauncherEntry. Others are difficult to find documentation for. We seem to have non-standard items in some Gnome applications, for instance, beginning with X-GNOME-

So the question is: where can I find information about all of these, including these extensions? Are there extensions for all desktop environments, like KDE, Xfce and LXDE as well? In other words, what I am seeking, is a complete desktop file reference. Before you answer, please check for duplicates in http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s05.html. Those are standard and that's a good reference for those. It is all those other types I'm interested in.