Seventeen years, that’s a long time to prepare a film, and it’s how long actress-director Maïwenn has spent on “Jeanne Du Barry,” which inaugurates the Cannes Festival today, a film that’s a nod to Sofia Coppola’s “Marie Antoinette” which was shown here in 2006, but also a nod to the past since real-life Marie-Antoinette became Louis XVI's wife, ending the courtesan's favored place in the court. It is thanks to "Marie-Antoinette"