Move most of scroll bar handling and consolidate styles into BAR_TYPE.
Move most of scroll bar handling and consolidate styles into BAR_TYPE.

This also clears the way for scroll bar gutters, see Bars.size().

tomfisher/tripleplay,joansmith/tripleplay,joansmith/tripleplay,tomfisher/tripleplay,tomfisher/tripleplay,tomfisher/tripleplay,joansmith/tripleplay,tomfisher/tripleplay,joansmith/tripleplay,joansmith/tripleplay