This implementation of <code>dragStart</code> will create a default
image that will be used during the drag. The image should be disposed
when the drag is completed in the <code>TableDragSourceEffect.dragFinished</code>
method.
Subclasses that override this method should call <code>super.dragStart(event)</code>
to use the image from the default implementation.
@param event the information associated with the drag start event
This implementation of <code>dragStart</code> will create a default image that will be used during the drag. The image should be disposed when the drag is completed in the <code>TableDragSourceEffect.dragFinished</code> method.
Subclasses that override this method should call <code>super.dragStart(event)</code> to use the image from the default implementation.
@param event the information associated with the drag start event