Issue 621: Dynamics should avoid cross-staff BarLines

Issue 621: Dynamics should avoid cross-staff BarLines
(e.g. GrandStaff, PianoStaff etc)

Dynamics usually have extra-spacing-width set to an empty interval so
that their placement does not cause other elements to shift.

With span bars, however, the resulting overlap is a worse cure than
the problem.  So this switches off the width-hiding setting of
extra-spacing-width inside of staff groups using span bars by default.
No extra space is allocated, so dynamics will clear the span bars only
narrowly, a reasonable compromise.