HorizontalHeaderView: Read `sort` role to determine if we should show sorting
Since the header lacks API to set this, we need to come up with a
different solution. This was the most minimal that I could get it.
Behaviour now is, if there's no sort role or it explicitly returns
undefined, we don't show any sort indicator. If there is, it is assumed
to be Qt::SortOrder
and we use it to show either the up or down arrow.