Bala and Mullein

Copperbox revision 800.

I've added SpacerMark as an alternative glyph type. This gives an easy mechanism to add fretboard diagrams to scores - a staff can be divided into two context one contains the melody as normal the other fretboard diagrams timed with spacer rests. This gives a useful separation between the fretboard diagrams and the melody - the diagrams don't have to be attached to a specific note (which would mean elaborating the glyph type with some annotation mechanism).