Remember Paul Williams? The ’70s music icon will be at the Gene Siskel Film Center to promote a documentary by Stephen Kessler, “Paul Williams Still Alive.” Kessler actually thought his idol was dead, but upon discovering him not only alive but still going strong, he decided to document Williams’ life today — back on track after a long battle with addiction. Both men will be at the 7:45 p.m. Saturday screening of the film at the State Street film center The documentary has a one-week run — through July 5.