Let make-music accept existing music or alists as source

Let make-music accept existing music or alists as source

This makes it possible to replace, say, a @samp{NoteEvent} in
@var{event} with a @samp{SkipEvent} (preserving all information) by
writing

(make-music 'SkipEvent event)


The first commit only changes the definition of make-music.  A second
commit is

Use make-music with music arguments

[thomasmorley651, 2013-09-25 22:28:38]

LGTM