Approximates cross-staff slurs in VerticalAxisGroup vertical-skylines. (issue 6498077)

[k-ohara5a5a]

I might have a test case for you at
http://www.mutopiaproject.org/cgibin/piece-info.cgi?id=1776

It seems you copy each slur into a "slur-stub", and from those keep only
the ones with "cross-staff" set. Then when figuring system skylines you
insert all Grobs with the slur-stub-interface into the skylines for each
staff.

Why not insert the original Slur into the skylines, if it has
"cross-staff" instead of the SlurStub.  For that matter, why not insert
all Grobs marked "cross-staff"?

all the cross-staff

http://codereview.appspot.com/6498077/