span-bar-engraver.cc: Sort staves in vertical order, orphans last (issue 6406051)

[graham]

If this isn't covered already, then could you add a regtest for this?

Mike presumably had some reason for removing it in the first place, so
if this doesn't produce a visible change in any regtests then it's
likely that somebody will make this change again in the future.

http://codereview.appspot.com/6406051/