TEN-MINUTE PLAY: Laura and Dean love each other. That has never really been the problem. What is the problem resists easy diagnosis - it lives somewhere in the silences, the small failures, the distance that grows between the couch and the loveseat. Structured as a theatrical sonata in eight movements, "Variations in the Key of Laura" is a chamber drama for two actors that finds the full weight of a relationship in a single living room, a coffee table, and a letter that might be better off unopened