Though he’s never been a contestant, Graham Colton is inextricably linked with “American Idol.” The heartland rocker—who has often been compared to a young Tom Petty—has toured with (and also dated) inaugural Idol Kelly Clarkson, and this year his 2007 single “Better Days” became the exit music for the latest season of the popular Fox talent show. Tonight, Colton brings his gentle radio rock (and his gentle good looks) to the Turner Hall Ballroom for an 8 p.m. concert.