Katharine Hepburn, one of the most iconic actresses of the 20th century, is the latest person to be featured in the National Portrait Gallery’s “One Life” gallery. Kate: A Centennial Celebration, which opened today, presents a multi-media approach to Hepburn’s life — photographs, video clips, her four Best Actress Oscar statuettes, a Brooks Brothers red turtleneck sweater, and a studio contract are all on display. The four statuettes, which Hepburn won for Morning Glory, Guess…