Also removes the side-effect that SCH_SHEET_LIST's
c'tor would sort the list (and write virtual page
numbers) anytime the starting sheet was the root.
Also, definitely don't build a SHEET_LIST (sorted or
otherwise) if you're not even going to use it.
Also don't build SCH_SHEET_LISTs on idle events. Better
to just always have the Next Sheet button enabled (we
already beep if you click it and there's no next sheet).
Also, use a SCREEN_LIST when you can. It's much cheaper
to create.
