PATCH: Use same fret-assignment code for FretBoards and TabStaff

[Carl Sorensen]

In response to a request from Patrick Schmidt on the tablatures list, I
became aware of the fact that we have two separate methods of finding
strings and frets given notes -- one for the tab-note-heads-engraver and one
for the fretboards-engraver.

This didn't seem to me to be a very good situation, so I rewrote the code so
that both engravers call the procedure specified in the context property
noteToFretFunction to get frets and strings from notes.

Please review the patch at

http://codereview.appspot.com/186268/show


Thanks,

Carl