A 26-week treatment, multicenter, randomized, double-blind, double dummy, placebo-controlled, adaptive, seamless, parallel-group study to assess the efficacy, safety and tolerability of two doses of indacaterol (selected from 75, 150, 300 & 600 µg o.d.) in patients with chronic obstructive pulmonary disease using blinded formoterol (12 µg b.i.d.) and open label tiotropium (18 µg) as active controls