Make -sourceList:objectValueForItem: an optional method on PXSourceListDataSource.

This is to allow for view-based table support.
Add PXSourceListDataSource method -sourceList:sortDescriptorsDidChange:.

This brings the API on-par with that of NSOutlineViewDataSource.
Add PXSourceListDelegate methods for view-based tables support.
Make PXSourceListDelegate on-par with NSOutlineViewDelegate.

Add missing delegate methods to PXSourceListDelegate which make sense
that are counterparts to those declared on NSOutlineViewDelegate.