Added transition lines for lyrics (issue 565750043 by address@hidden)

[davidgrant . no]

Reviewers: ,

Message:
I've been working on this feature I would like to propose for LilyPond,
and I wonder how I should proceed (request feedback on users group,
discussion here, or something else?). I'm aware of being very much a
beginner here, but I'm tentatively posting this hoping for feedback
either on the process or the code itself.

I do have some questions:
- Copyright notices at the top of new files: Should I always be adding
my own name (as I have done so far), even though the contents is based
on existing code? I'm assuming the fact that there _is_ a notice in the
files is the main goal, rather than the contents itself. Of course, I
don't want to be taking 'credit' for code written by others, but I also
don't want to be attributing anything to them that they wouldn't want to
touch with a ten foot pole.
- Everything is bundled together here - the feature itself, regtests and
docs. Should this be broken apart into separate commits?

Thanks!

Description:
Added transition lines for lyrics

A transition line (my suggested term) is drawn as an arrow from one
syllable to another, and indicates a gradual change of vowel (see Gould
pp. 452--453).

I propose '->' to be used between lyric syllables (similarly to hyphens
are input).

Please review this at https://codereview.appspot.com/565750043/

Affected files (+466, -59 lines):
  M Documentation/changes.tely
  M Documentation/music-glossary.tely
  M Documentation/notation/vocal.itely
  A input/regression/lyric-transition.ly
  A input/regression/lyric-transition-broken.ly
  A input/regression/lyric-transition-offset-syllable.ly
  A input/regression/lyric-transition-padding.ly
  A input/regression/lyric-transition-right-margin.ly
  A + lily/include/lyric-transition.hh
  M lily/lexer.ll
  A lily/lyric-transition.cc
  M lily/parser.yy
  A + lily/transition-engraver.cc
  M ly/engraver-init.ly
  M scm/define-event-classes.scm
  M scm/define-grobs.scm
  M scm/define-music-display-methods.scm
  M scm/define-music-types.scm
  M scm/safe-lily.scm