The Pennsylvania Academy of the Fine Arts in Philadelphia – the first art museum in America – displayed the work of a different kind of artist on a warm spring night when Ani DiFranco played the Artists Den stage. Before a packed gallery in the Academy’s Historic Landmark Building, which opened in 1876 for the nation’s Centennial Exposition and World’s Fair, the prolific, fiercely independent folk singer-songwriter gave a special performance of both fan favorites and new songs from her album, Red Letter Year.