on-mouse-over in graph-pasteboard<%>

• (send a-graph-pasteboard on-mouse-over a-los) -> void
     a-los : (listof snip%)

  This method is called when the mouse passes over any snips in the editor. It is only called when the list of snips under the editor changes (ie, if the mouse moves, but remains over the same list of snips, the method is not called). Also, this method is called with the empty list if the mouse leaves the pasteboard.