Police sources say that the woman found dead in her West 22nd Street apartment was killed by her boyfriend after she broke up with him. The Post reports that 26-year-old Margaux Powers (pictured, with her father) had "canceled plans to attend a family gathering Thursday" so she could break up with Jonathan Smith, 34. Powers and Smith, a chef who had been recently laid off, lived together in the Chelsea apartment. Police believe Smith slit......