gdm-config should skin post-2.20 gdms

[broder]

Right now our debathena-gdm-config package won't make any attempt to skin post-2.20 gdms, which no longer use the XML based themes.

There's a fair amount of discussion about this in #369.

[broder]

Oh - I should also mention that, since we can only change the background image and not much of the rest of the layout, our old-style theme probably won't apply particularly well to the New World Order.

This may be an excellent opportunity to find a real designer to do our design work, as opposed to one of us developers doing it badly.