Split ItemTree megaclass into:
- ItemTree - concerned with drawing the virtualized table container and
column interaction
- ItemTreeRowProvider - provides rows and issues notifications for
render updates
- ItemTreeRow and subclasses - contains row-specific data and rendering
logic
- CollectionViewItemTree and its accompanying classes - a version of
ItemTree that renders items attached to a given Collection or
CollectionView (CollectionTreeRow).
Various improvements in logic and rendering, separation of concerns.
