Computes and returns an image to use for dragging.
Override this to return a custom image. dragRows represents the rows participating in the drag. dragEvent is a reference to the mouse-down event that began the drag. dragImageOffset is an in/out parameter.
This method is called with dragImageOffset set to NSZeroPoint, but it can be modified to reposition the returned image. A dragImageOffset of NSZeroPoint will cause the image to be centered under the cursor.