Long a cult favorite, painter John Wesley receives an overdue first U.S. retrospective on view through November at New York's P.S.1 Contemporary Art Center. To mark the occasion, Dave Hickey offers an appreciation of the pop eccentric's wry and whimsical four-decade career.WHEN POST-GLOBAL-WARMING anthropologists begin paddling through the streets of Manhattan in search of visible evidence that this republic was, in its tone and temper, the cosmopolitan democracy that it purported to be, one can only hope that the earnest scientists will stumble across a trove of John Wesley's paintings in some