Floor filler? The turnaround has been astonishing. Guitar Hero launched in 2005, its first set-list composed extensively of cover versions, with the music industry's nostrils too busy sweeping lustily over cisterns to pay it much heed. Developer Harmonix was virtually required to shuffle up to record label doors, cap in hand, begging for whatever scraps of licensed music were lying around. $2 billion and 25 million sales later, someone else is now wearing the trousers in this ...