feta-accordion.mf, convertrules.py, musicxml2ly.py: sanitize accordion symbol names (issue1908041)

[Carl . D . Sorensen]

Looks good, with the exception of the version being wrong for the
convert rule.

Carl



http://codereview.appspot.com/1908041/diff/2001/3002
File mf/feta-accordion.mf (right):

http://codereview.appspot.com/1908041/diff/2001/3002#newcode117
mf/feta-accordion.mf:117: fet_beginchar ("accordion register freebass",
"freebass")
I made these changes, and then didn't apply them, because I think the
"base" that is intended is not "bass", but the base for a symbol that
will be completed by adding the dots.  So I'm not sure that we want to
move to "bass", but you're the accordion expert, so if you like it, go
for it.

http://codereview.appspot.com/1908041/show