UNISUS: A Phase 3, Prospective, Multicenter, Double-blind, Double-dummy, Randomized, Active-controlled, Parellel-group, Group-sequential, Adaptive, Event-driven Study to compare Efficiacy, Safety and Tolerability of Macitentan 75mg Versus Macitentan 10mg in Patients with Pulmonary Arterial Hypertension, Followed by an open-label Treatment Period with 75mg Macitentan