Work in progress for chord symbol spacing.

This expands the containing segment to allow space for the chord symbol.
Further work required to allow a chord symbol to overlap segments that
do not have their own chord symbol.
Allow harmony to span multiple segments.

Also add a style setting for minimum chordname spacing.
Add stretch to notes under a long chord symbol.

Also, allow chord symbols to overlap if separated vertically.
Remove debug code and update some comments
Use a tight bounding box for Harmony layout.

Also normalise chord names to lower case when matching typed input.
Merge pull request #262 from softins/chord-symbol-layout

Chord symbol layout