Properties to control placement of accidentals in KeySignatures (issue 6461085)

[dak]

Will the added generality be enough?  Should we not try to come up with
some sort of exception list instead, like we do for mapping chords to
fretboards?

I think the principal aim should be to be able to avoid having to write
overrides in the music itself: changing key/scale and clef should be all
that is needed.

http://codereview.appspot.com/6461085/