Even during his ’70s prime, Stevie Wonder was never much of a road-warrior, and these days, with his stardom well cemented, the perennially smiling soul singer prefers to do quick television appearances instead of laborious tour dates. This year, however, the happy one mercifully made space in his selective schedule for an ultra-rare Milwaukee appearance, so catch his 7 p.m. show at the Marcus Amphitheater tonight, because he likely won’t be back for a very long time.