“Heartbeats,” a fluffy romantic soufflé, is introduced by an epigraph from the French poet Alfred de Musset: “The only truth is love beyond reason.” Well, I suppose if you’re as young and as besotted with cinematic dreams as Xavier Dolan, the 21-year-old Québécois who wrote, directed and edited this film, and also served as its art director and costume designer, that slogan makes a crazy kind of sense.