Almost finished serialising stuff.

Lots of properties of things are derived or only necessary in parsing phase. When I finish simple deserialisation I'll make StemParsed and NoteParsed sub classes to clean parsing stuff out of Stem and Note classes. That seemed to work ok for bars and lines. I think I'll need a ScoreParsed too.

Properties in the base class can be simple get/set (or cached get) and derived classes can derive value each time (which is good when editing).