A phase III, 52-week, multinational, multicenter, randomized, double-blind, 2-arm parallel group study comparing efficacy, safety and tolerability of the fixed dose triple combination of beclomethasone dipropionate plus formoterol fumarate plus glycopyrronium bromide (CHF 5993) with the fixed dose dual combination of beclomethasone dipropionate plus formoterol fumarate (CHF 1535), both administered via pMDI in subjects with chronic obstructive pulmonary disease (COPD)