A subclass of NSSplitView that adds auto-saving of the sizes of its subviews.
- awakeFromNib, send the split view
setPositionAutosaveName:(NSString *)name. It works just like the NSWindow
*In the latest version of the OmniFrameworks, it also adds a method the delegate can implement to do something when the divider is double-clicked. (Like completely collapse a particular subview)