MusicXML: Convert fretboards from MusicXML to Lilypond

MusicXML: Convert fretboards from MusicXML to Lilypond

In MusicXML, fretboards are inside <harmony> elements, assigned
to a particular staff, while in lilypond the \fret-diagram is a
text markup assigned to a particular note. So we run into the same
problems as we do with dynamics (i.e. we need to find a proper
note to assign the element to, which is not very robust). For
single-voice harmony, this works quite well so far.