Neume

Copperbox revision 1245.

I've implemented a new traversal for changing Duration to (Maybe Duration) for LilyPond. In the end I decided not to use a traversal based on the last post (i.e. separating shape and contents), and I'm using one that tracks an index instead so it knows when it is at the first note in a bar.

The new traversal is polymorphic on glyph - whereas the previous one only handled 'standard' glyphs.