HTC has unveiled the latest edition to its mid-range handset range: The HTC Desire 820. It's big. The 820 is a successor to the Desire 816 (us neither) and is powered by a Snapdragon 615 processor. The screen is a large-for-midrange 5.5". Crucially too the handset has HTC's "Boomsound" dual front-facing speakers. On the rear,…