An outline view that can be scrolled without it having fits.

This is basically `RBLTableView`, but as an outline view. I literally copied and pasted `-[RBLTableView scrollRectToVisible:]`, because of course I can’t compose that method cleanly without mucking with the runtime.

I could optionally extract it into an `NSView` category as `-rbl_scrollRectToVisible:` and call it in both locations, which would avoid the duplication.