Fix issue 75: Allow multiple concurrent slurs (issue4643067)

[pkx166h]

Hello, I get an error when I try to make:

--snip--
[/home/james/lilypond-git/out/share/lilypond/current/scm/document-music.scm]
[/home/james/lilypond-git/out/share/lilypond/current/scm/document-type-predicates.scm]
[/home/james/lilypond-git/out/share/lilypond/current/scm/document-identifiers.scm]
[/home/james/lilypond-git/out/share/lilypond/current/scm/document-backend.scm
error: define-grob-properties.scm: cannot find interface for property:
slur-id
make[1]: *** [out/internals.texi] Error 1
make[1]: Leaving directory `/home/james/lilypond-git/Documentation'
make: *** [all] Error 2
address@hidden:~/lilypond-git$

---snip--

Jame

http://codereview.appspot.com/4643067/