Utilise grammar-parsed chords

[martijnversluis]

we're parsing chords, but when creating a song we're cramming it into a string because that's what it expects. Instead, we should be able to incorporate the parsed chords into the song. We should either differentiate between a chord object and a chord string, or ensure a chord is always a Chord object (but that adds parsing overhead when you might not even need the parsed chord)