[Patch] Fix #1333: beamed multi-note acciaccaturas (issue3169041)

[v . villenave]

Reviewers: ,

Message:
Greetings everybody,

here's a patch (more of a workaround, really) to address
http://code.google.com/p/lilypond/issues/detail?id=1333
(aka Lily's inability to print beamed acciaccaturas with a slashed
stem).

Whilst not very elegant, it does allow to handle multi-note grace
expressions differently, which might be interesting in the future.

Thanks for having a look!

Description:
Fix #1333: beamed multi-note acciaccaturas with a slashed stem

This commit modifies the def-grace-function macro to make it
aware of wether its argument is a single note or a SequentialMusic
expression.  The appropriate rules are looked up and applied
accordingly; if a corresponding rule has not been defined, then the
standard GraceMusic rules are applied instead.

Where as single-note acciaccaturas are printed using a font glyph,
this implementation uses a modified stencil to print a slash on the
first stem of several beamed notes.  It allows for a pure-Scheme
implementation, albeit arguably not very elegant.

Please review this at http://codereview.appspot.com/3169041/

Affected files:
  A input/regression/grace-beam-stroke.ly
  M ly/grace-init.ly
  M ly/music-functions-init.ly
  scm/music-functions.scm