* ly/music-functions-init.ly (parallelMusic): new music function for parallel

* ly/music-functions-init.ly (parallelMusic): new music function for parallel
music entry.

* scm/define-music-properties.scm (all-music-properties): new 'void music property

* scm/music-functions.scm (ly:music-message): make public, to be
used in music function definitions

* scm/lily-library.scm (collect-music-for-book): discard music
when the 'void property is set.