The vast Valentino exhibit that opened today at London’s Somerset House is a departure from the sometimes stuffy world of haute couture. Envisioned as a reverse fashion show, visitors walk down a central runway to view the 130 gowns on mannequins that make up the audience. Grouped by color to show the decade it was made, the mannequins stand among chairs with name tags that read Elizabeth Taylor, Grace Kelly, and so on. Here’s a behind-the-scenes video from Crane.tv…