I've set EnableColumnReorder to true in my NTreeList.  Been looking (ColumnNotify, HeaderNotify, and HeaderItemDragAction) but I can't find an event that gets fired when the user reorders the columns with the mouse.  What's the recommended way to know that the user has done this and the columns have changed order?  On a grid there is a ColumnDisplayIndexChanged event.  What's the way to know this in an NTreeList?