CHRISTMAS IN Midtown Manhattan is always more than just a holiday shopping tradition. It becomes the world's preeminent showcase of visual merchandising at its grandest and most enchanting form. For decades now, beginning in mid-November, Bloomingdale's, Macy's, Saks Fifth Avenue, and other department stores on and around Fifth Avenue outdo each other in transforming their windows into elaborate, magical wonderlands. It's the stuff that visual merchandisers dream of.