NSScrubber family is unimplemented (empty classes / stub returns)

[DTW-Thalion]

The NSScrubber family is present as skeletons only:

• NSScrubber (@interface NSScrubber : NSView with no declared methods or ivars), NSScrubberItemView and NSScrubberArrangedView are empty @Implementations.
• NSScrubberLayout and NSScrubberLayoutAttributes declare methods but every one returns nil / NO / NSZeroRect / NSZeroSize / 0, and +[NSScrubberLayoutAttributes layoutAttributesForItemAtIndex:] returns nil so a populated instance cannot be constructed.

A real implementation would need the NSScrubberDataSource and NSScrubberDelegate protocols, the NSScrubber properties (dataSource, delegate, selectedIndex, mode, itemAlignment, scrubberLayout, ...), the data methods (register.../reloadData/itemViewForItemAtIndex:/insert/remove/move), flow and proportional layouts, and the item-view subclasses.

Filing to record the state; this is a sizeable feature rather than a fix.