PARIS — On Saturday afternoon, people trickled across the large plaza in front of the Hôtel de Ville. Pausing to admire city hall’s façade, some snapped pictures while others gathered around a group of break-dancers before shuffling off. Meanwhile, tucked away in a far corner of the plaza, an eclectic group was hanging painted banners that read “Modèles” (“Models”), “Modèles d’art: Poser c’est un métier!” (“Live modeling: it’s a job too!”), and the name of their organization, Collectif des Modèles d’Art de Paris (Art Models Collective of Paris).