A 52-week, Randomized, Double-blind, Double-dummy, Placebo- and Active- Controlled (Roflumilast, Daliresp® 500µg), Parallel Group, Study to Evaluate the Efficacy and Safety of Two Doses of CHF6001 DPI add-on to Maintenance Triple Therapy in Subjects With Chronic Obstructive Pulmonary Disease (COPD) and Chronic Bronchitis.