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.
They don't define a KiCad string class, so the header file name was
somewhat misleading. But the fact that they didn't match definitely
made coding more difficult.
It's tempting to say that we don't need to exclude filename chars
from symbols, but we might regret that decision down the road. Better
to just escape them.
Fixes https://gitlab.com/kicad/code/kicad/issues/8694
