[Pseudo-patch] Add support for HairpinText indications. (issue3509041)

[v . villenave]

Reviewers: ,

Message:
Greetings everybody,

I am currently trying to understand how Grobs are created and handled,
and here's my first attempt at fiddling with them. Needless to say, it
doesn't work (exits with a segfault) but at least it doesn't seem to
break existing things.

My eventual goal would be to create a new HairpinText grob that would be
printed above or below hairpin crescendos, a bit like
http://lsr.dsi.unimi.it/LSR/Item?id=233 but in a native, Grob-ish way.

For that purpose, I've created a new 'mixed span-type (I think I got
this sort of covered now) and a new Grob (which is, for now, pure
nonsense).  I could really use some pointers (no pun intended) as to
where to go from this point, assuming my approach is correct in the
first place, and this feature is worth implementing at all (which can be
argued upon).

Anyways, please have a look and let me know! Thanks!

Description:
Add support for HairpinText indications.

This first attempt, although unsuccessful, aims to
implement native, tweakable text indications that
are sometimes found above or below Hairpin objects.

(See LSR snippet #233 for an example.)

Please review this at http://codereview.appspot.com/3509041/

Affected files:
  A input/regression/hairpin-text.ly
  M lily/new-dynamic-engraver.cc
  scm/define-grobs.scm