“Saving Mr. Banks” is Disney’s latest attempt to tug at our wide-eyed hearts, but there isn’t enough sugar in the world to make this medicine go down. With apologies to every actor in the film, all of whom perform wonderfully, this is perhaps the most crass, mean-spirited, propagandistic release in Disney’s storied history — precisely because this is once again Disney rewriting its own history and exploiting P.L. Travers, creator of “Mary Poppins,” for a second time. Walt Disney (Tom Hanks) tried for 20 years to convince Travers (Emma Thompson) to let his studio adapt “Mary Poppins” for the big screen.