f19 and f20: clean up admin-tools group, use it for LXDE, Xfce and MATE
    
    The 'admin-tools' group contains several utilities that would actually be
    useful for many desktops, but it got lost in the shuffle between F17 and
    F18. This commit drops gnome-packagekit from it (other desktops don't
    necessarily want it, they may choose apper or yumex) and adds abrt-desktop
    (which all the desktops were adding separately), and adds the group to the
    LXDE, Xfce and MATE desktops: I am not yet sure whether the KDE, Cinnamon
    or Sugar desktops would want it, and I'm pretty sure GNOME does not. For
    F20 only, the commit also drops all the 'optional' stuff from the group,
    as optional is pretty much a no-op now anyway.