Make core Xfce components mandatory
    
    This makes core Xfce desktop packages mandatory. That means
    the group they are in (xfce-desktop) will not be considered to be
    'installed' if they are removed, and importantly, image composes
    will fail (as opposed to 'succeeding', but with the packages left
    out) if they are missing or not installable. That's the main
    benefit here.
    
    Amendments to the list welcome, this was just my best guess at
    what constitutes the really core packages.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>