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>