Forcing arrangement of objects even when fetch request has no results

- NSArrayController doesn't arrange anything when it's fetch request returns no results, it's possible to have arbitrary groups so we need to force arrangement to occur;
- Additionally, we need to override arrangedObjects to return whatever it is we produce in arrangeObjects: since we don't know what is calling arrangeObjects:.  Will need to monitor this to be sure there is no performance tax or loss of functionality.
- At this point it may be better to stop sub-classing NSArrayController completely :(