Seventeen years ago, Gloria Cannon had an idea, to preserve “The Barbershop,” a longtime Southampton gathering place for Black East Enders. On Saturday, Juneteenth, after years of planning, fund-raising, construction, and a few setbacks, her vision will be realized with the grand opening of the Southampton African American Museum at 245 North Sea Road, the site of the former barbershop built by Emanuel Seymore in the 1940s.