Part staves (and barline type short)

[rpatters1]

It seems to me that short should not (only) be a barline type. It should also be a staff feature. The "shortness" must have the ability to apply to any barline type, with the exception of tick and noBarline. It should also have the ability to apply to repeats. Dorico mentions the use of short barlines in plainchant. I suppose in this context the other barline styles would be full length. Dorico also has a shortTop barline type, which we should consider adding.

Thinking about this led me to realize that staff features are sorely needed anyway. I can't find any current way to specify a number of staff lines (e.g., for a percussion staff), nor the spacing between lines. (Among many others, I would like MNX to be able to encode Orff Schulwerk scores, and those make very creative use of staff lines.)

A way to add staff features that occurs to me immediately would be, instead of a staves count in the part object, to have a staves array. Then we could add whatever staff features are needed.