“The Dining Room” is a play that lacks a linear plot. Instead, it uses seven actors to play 50 parts in 19 stories that take place in the dining room of an upper middle class home. Winterport Open Stage, returning after a three-year hiatus, vividly brings A.R. Gurney’s 1982 two-act play to life at the Samuel L. Wagner Middle School in Winterport. It is funny, witty and frothy, just what audiences have come to expect from the company over the past three decades.