This method is called when the user starts interactively dragging snips (the ones that are selected; see find-next-selected-snip), after can-interactive-move? is called to verify that the move is allowed. The after-interactive-move method is guaranteed to be called after the move has completed. All of the selected snips will be moved. The mouse event that started the move (usually a button-down event) is provided.
See also interactive-adjust-move.