A proposal to build Boston’s tallest residential building in the Back Bay is generating objections from neighbors who argue the project would worsen traffic problems, darken Copley Square with shadows, and skimp on public art and affordable housing. Several groups have told the Boston Redevelopment Authority that those issues must be addressed before Simon Property Group’s 47-story tower at Copley Place is permitted. In response Simon late Thursday said it will double the number of affordable units in the $500 million tower and buy a nearby building where it will create 35 affordable units.