* BrushChannelSet is now a pure collection. This
eliminated the annoying brush.channels.channels
paths, and also makes library overriding work.
* Now sure I've done this correctly. Struct extended
collection properties are a bit confusing.
* Move more dyntopo settings to brush channels
* Implemented the unprojected radius hack in
the new brush system. I'm not really happy
with it, but doing it properly is going to
take some thought.
