Improves horizontal spacing of axis groups that SpanBar grobs traverse. (issue 4917046)

[mtsolo]

Reviewers: ,

Message:
Hey all,

I've been thinking about Neil's comment regarding a better interface for
cross-staff stems.  While this patch has nothing to do with stems per
se, it lays down the framework for dealing with cross-staff stems via a
test-case with SpanBar grobs (which are like cross-staff stems insofar
as they span between staves and take their alignment anchors from grobs
with pure height approximations).

In doing so, it fixes issue 910 (regest added to show this) and gets rid
of an unreported but bad problem that SpanBar pure heights are either
empty or not depending on the common refpoint being used.  In theory, a
pure height should return the same value for .length () every time.

Cheers,
MS

Description:
Improves horizontal spacing of axis groups that span bars traverse.

Please review this at http://codereview.appspot.com/4917046/

Affected files:
  A input/regression/span-bar-allow-span-bar.ly
  M lily/align-interface.cc
  M lily/include/align-interface.hh
  A lily/pure-from-neighbor-engraver.cc
  A lily/pure-from-neighbor-interface.cc
  M lily/span-bar-engraver.cc
  A lily/span-bar-stub-engraver.cc
  M lily/span-bar.cc
  M ly/engraver-init.ly
  M scm/define-grob-properties.scm
  M scm/define-grobs.scm
  M scm/output-lib.scm