Rejig how we count the g-i-s screens we'll encounter
    
    Trying to keep track of what these magic numbers mean is really
    getting messy, so let's do it a bit more explicitly, using the
    page names g-i-s uses internally, and lots of comments. This
    should make it clearer and more maintainable when stuff changes.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>