After years of effort, it looks like it’s really going to happen this time: Downtown Crossing will be revitalized as a center of commerce, yes, but also as a desirable place to live. Construction of Millennium Tower , a 60-story glass structure with 442 residences, is underway at the area’s most prominent intersection. And next door, in the beautiful 1912 Burnham Building that once housed the original Filene’s, work is underway on a Roche Bros. supermarket.