Capo chords and parens

This is a proof-of-concept for two enhancements to chord symbols:

1. Display of capo chords in brackets after the main chord, when capo is non-zero. For the time being, the capo position is stored as a style setting in the Chordname section of General styles. This can be replaced in the future when a more general capo setting is implemented in a similar way to key signatures.

2. Display of left and/or right bracket for a chord, so that one or more chords may be indicated as optional or alternative. This uses _leftParen and _rightParen properties in the Harmony object, and for now stores these settings in the MSCX file as <leftParen/> and <rightParen/>. This can be modified if required.