Lincoln Center marks a half-century of existence with this three-day paean to the musical, America's most fanciful contribution to the arts. The 20 films on slate represent six decades of gotta-dance pageantry, with no bagatelles to be seen. Singin' in the Rain jump-starts a TGIF schedule that includes Bob Fosse's Weimar-era masterwork Cabaret and another midnight opportunity to memorize Purple Rain. Come Saturday, Busby Berkeley's madcap flair is on full-scale display in The Gang's All Here, while luck delivers twofold for Frank "Pal Joey" Sinatra with Rita Hayworth and Kim Novak. And Sunday is simply hubba-hubba worthy, with Marilyn Monroe in Gentlemen Prefer Blondes — or, if you prefer, Gene Kelly, as he goes balletic in New York and Paris.