Lettered tablature patch

[Trevor Daniels]

I posted my first patch to Rietveld last night, but nothing seems to 
have been cc'd to the list.  Not sure what I did wrong.  It's at 
http://codereview.appspot.com/164063.  Here's what it does.

This is the first patch of a series aiming eventually to enable 
Baroque lute tablature to be produced.

This first patch enables an option to indicate fret numbers with 
letters.  The option is turned on by setting the context variable 
tablatureFormat to fret-letter-tablature-format.  The letters are 
taken sequentially from the alphabet by default, but the first n of 
these may be overridden by setting the context variable fretLetters 
to a string of n length.  For example, the third fret on a Baroque 
lute is indicated by "r" rather than "c", as "c" can easily be 
confused with "e".  (Actually it was originally a Greek gamma, but 
"r" is usually used).  So, for a Baroque lute, fretLetters would be 
set to #'("a" "b" "r").

Following a suggestion by Neil I have also made the whiteout behind 
all fret numbers optional.  This is controlled by the 'whiteout 
property of TabNoteHead.  The default is #t to preserve the current 
behaviour.

As this is my first attempt at modifying LilyPond code I'm sure 
there are many things that could be improved.  Comments welcome.

Trevor

ps I've not included an addition to changes nor any documentation. 
I'll do that after this approach (the three new properties) has been 
approved.