Adds glissando stems to Lilypond. (issue4661061)

[mtsolo]

Reviewers: ,

Message:
I have a functional version of this feature up with a regtest.
I think the input syntax works, but it could be cleaner.

All suggestions are welcome!  Off now to typeset crazy glissandi...

Cheers,
MS

Description:
Adds glissando stems to Lilypond.

Please review this at http://codereview.appspot.com/4661061/

Affected files:
  A input/regression/glissando-stem.ly
  M lily/glissando-engraver.cc
  M lily/include/stem.hh
  M lily/music-sequence.cc
  M lily/note-spacing.cc
  M lily/staff-spacing.cc
  M lily/stem-engraver.cc
  M lily/stem.cc
  M ly/music-functions-init.ly
  M scm/define-grob-properties.scm
  M scm/define-grobs.scm