Thursday, 8.35pm, Showcase A seventh season for the world’s most lovable serial killer, Dexter Morgan (Michael C. Hall) is coming to you so soon after its US debut that no previews were available by press time. From memory, Dex will have some explaining to do. You’ll recall that at the end of last season, in This Is the Way the World Ends, Dexter was going about his revolting business when he heard a gasp. He looked up and there stood his sister. ‘‘ Oh God,’’ quoth he. Quite.