MusicXML export of unrecognized chords

A chord description object is now generated for each (unique) unrecognized
chord, and MusicXML tags are generated during parsing for inclusion in the
chord description.  This allows unrecognized chords to be exported to
MusicXML using the same mechanisms as recognized chords.
MusicXML import of unrecognized chords

A new method is provided generate a chordname directly from a MusicXML
harmony object.  This chordname is then parsed and rendered just as for a
chordname typed directly into the score.