YOU might not think a movie about a fellow who lends his rooms to the married executives of his office as a place for their secret love affairs would make a particularly funny or morally presentable show, especially when the young fellow uses the means to get advanced in his job. But under the clever supervision of Billy Wilder, who helped to write the script, then produced and directed "The Apartment," which opened at the Astor and the Plaza yesterday, the idea is run into a gleeful, tender and even sentimental film. And it is kept on the side of...