NEW YORK - Ken Kaess, president/ceo of Omnicom's DDB Worldwide, has died aged 51 at his Connecticut home. He had been suffering from cancer and had been on medical leave since November.    Kaess, who had worked in the industry for around thirty years, twice served as chairman of the American Association of Advertising Agencies. He was also a driving force behind the creation of New York's annual Advertising Week event.