Xubuntu-artwork should only recommend Xubuntu-artwork-usplash

When I install an ubuntu box, sometimes users want a blue theme instead of the Human one. I then tend to download a metacity from the web and put them the Xubuntu icon theme and gtk theme, which are available in xubuntu-artwork.

It would be good that this package only recommends xubuntu-artwork-usplash (and xubuntu's gdm theme ?) so that one can install our gtk and icon theme with --without-recommends and without modifying their usplash and gdm theme, so that they don't need to restore them afterwards (our new GDM rocks though, but honnestly, our usplash isn't astoundingly sexy).