Recommend gnome-session-xsession rather than hard-require it
    
    This makes it possible to avoid installing gnome-session-xsession, which
    in turn pulls in xorg-x11-server-Xorg. It's still going to be pulled in
    to the default install as Recommends are installed by default.
    
    The motivation for changing this is that there are some arm devices that
    only work with Wayland and where Xorg is unsupported, as requested by
    pbrobinson.