* scm/display-lily.scm: new file. Define a `display-lily-music'

* scm/display-lily.scm: new file. Define a `display-lily-music'
function, that displays the music expression given as an argument,
using LilyPond notation.

* scm/define-music-display-methods.scm: new file. Implementation
of display methods for each music type.

* ly/music-functions-init.ly (displayLilyMusic): new function for
displaying music with LilyPond notation.

* scm/markup.scm: remove obsolete debugging code (for printing
markups with LilyPond notation).

* scm/define-music-types.scm (music-name-to-property-table):
* scm/clef.scm (supported-clefs): export, in order to be accessible
from the (scm display-lily) module.