This method is called when the user starts interactively dragging snips (the ones that are selected; see find-next-selected-snip). All of the selected snips will be moved. If #f is returned, the interactive move is disallowed. The mouse event that started the move (usually a button-down event) is provided.
See also on-interactive-move, after-interactive-move, and interactive-adjust-move.
Returns #t.