fix performance issue hiliting large lists

Thanks for the useful selector!

We found that even on a fast machine, it was very slow to hilite with mouseover.

The mousemove event was doing a very heavy JQuery select every time the mouse moved a single pixel.

This is a quick fix so that the mousemove is only called once per DOM element - really, it is a hack to convert the mousemove to behave like a mouseenter instead.  

Although this fix works, it is a bit quick and dirty!

AP
John