New engraver for braces (issue4807053)

[bordage . bertrand]

Reviewers: ,

Message:
Hi!

This patchs allows to add braces the same way as arpeggios.
Very useful for complex organ music.
Check this out, there is several examples:
http://imslp.org/wiki/12_Pi%C3%A8ces_pour_Orgue_(Gigout,_Eug%C3%A8ne)

For the moment, the new engraver is a clone of Arpeggio_engraver.
But I would like to add a command to attach text directly to braces (see
IMSLP's
score). Since there's many cases for the text attachment points, I think
a new
engraver is required.

Done with Mike's help.

The regression test is temporary.

Regards,
Bertrand

PS: Li Jie Wong requested this last year :
http://lists.gnu.org/archive/html/lilypond-user/2010-11/msg00361.html



Please review this at http://codereview.appspot.com/4807053/

Affected files:
  A input/regression/braces.ly
  M lily/arpeggio.cc
  A lily/brace-engraver.cc
  M lily/include/arpeggio.hh
  M lily/span-arpeggio-engraver.cc
  A lily/span-brace-engraver.cc
  M ly/engraver-init.ly
  M ly/property-init.ly
  M scm/define-context-properties.scm
  M scm/define-event-classes.scm
  M scm/define-grob-interfaces.scm
  M scm/define-grob-properties.scm
  M scm/define-grobs.scm
  M scm/define-music-display-methods.scm
  M scm/define-music-types.scm
  M scm/safe-lily.scm