When we last saw Celine and Jesse, at the end of “Before Sunset” (2004)—the second film in Richard Link later’s trilogy about a French-American romantic alliance—they were in Celine’s Paris apartment, and Celine (Julie Delpy) was softly swaying to Nina Simone, while Jesse (Ethan Hawke) watched her, enchanted, and missed his flight home to his wife and child in New York. That final scene delivered a promise that the two would be a couple at last, a promise that has been fulfilled, for better and for worse, in the third film in the trilogy, “Before Midnight.” It’s been a long journey. The on-again, off-again relationship began in 1995, with the first film, “Before Sunrise,” when the characters were in their early twenties. In that movie, Jesse, a footloose American, meets…