Automatic LyricExtenders (issue 313240043 by address@hidden)

[perpeduumimmobile]

Reviewers: ,

Message:
For the sake of completeness: deserves an entry in changes.tely (TBD
when the syntax discussion is over that I expect to start some time
soon...)

Description:
Automatic LyricExtenders

As written and communicated by Knut Peterson.
For reference, see the threads
  http://lists.gnu.org/archive/html/lilypond-devel/2016-12/msg00223.html
  http://lists.gnu.org/archive/html/lilypond-devel/2016-12/msg00121.html
  http://lists.gnu.org/archive/html/lilypond-user/2016-12/msg00523.html
  http://lists.gnu.org/archive/html/lilypond-user/2016-12/msg00303.html
(sorry, this discussion grew quite long in the meantime).

This is a genuine Request For Comments at the current point in time.

Please review this at https://codereview.appspot.com/313240043/

Affected files (+235, -188 lines):
  M Documentation/learning/common-notation.itely
  M Documentation/notation/vocal.itely
  M lily/extender-engraver.cc
  M lily/lyric-extender.cc
  M ly/music-functions-init.ly
  M scm/define-grob-properties.scm
  M scm/define-grobs.scm
  M scm/music-functions.scm