Showhouses always have a whiff of unreality about them. After all, they're meant to let designers create spectacular spaces unfettered by concerns about price tags or finicky clients. The Boston Design Center's second annual Dream Home, on view through July 31, is no different. But this year, the fanciful touches - sliding lacquered teak cabinets, bath vanities with sinks so shallow they're almost invisible - have here and there been tempered by designs that feel a bit more down to earth.