Better approximations for cross-staff slurs

Better approximations for cross-staff slurs

SlurStub grobs are used to approximate the vertical skylines of
cross staff slurs by using control points generated by pure heights
instead of heights. Depending on how far off the actual translations are
from the minimal translations, these control points will be more or less
viable. A SlurStub is generated for every VerticalAxisGroup on which
a slur has encompass objects and only those residing on the extremal
UP and DOWN vertical axis groups get vertical skylines.