THERE'S a new power behind the throne in Zimbabwe: Grace Marufu. At a lavish wedding ceremony which went on all weekend of August 17-18, ident Robert Mugabe's former secretary introduced to the Zimbabwean people his wife. No doubt, the wedding bash be talked about for years, but right now ple are more interested in the political ence 31-year-old Marufu could wield in this southern African state, directly and through her new husband.