A N.Y. developer unveiled plans for a $200 million housing-retail development in Downtown Crossing, a milestone in the city's effort to revitalize the shopping district. Midwood Management Corp., which has acquired several properties in the neighborhood, said it plans to build 200 units of luxury apartments, in addition to three floors of retail space at the corner of Bromfield and Washington Sts. The company which plans to file plans with the Boston Redevelopment Authority, said it hopes to begin construction in 2010.