Add mensurstrich bar type

Define the bar type -span| to create span bars without visible in-staff bar lines.

Known issue: At a line break, the staff lines and span bar do not meet. I spent a few hours trying to fix it, but it's complicated, I don't want to make changes to StaffSymbol and SpanBar without understanding them better, and I don't want to spend more time on it now. I propose creating a ticket.

	Bottom line of upper staff
	Top line of lower staff