Cocoa NSOutlineView version of diffable data sources in iOS 13
Initialize a data source and send it its own NSOutlineViewDataSource and NSOutlineViewDelegate to pass through the calls.
When you call applySnapshot() on the diffable data source instead of reloadData(), it appropriately animates all the cells to their new locations.
- Perhaps honor the
responds(to: selector)stuff in the data source. I began this, but it wasn't necessary, so I punted. - Passthrough implementations of all the remaining delegate methods