Shanling Q2 port v2.0

Merge the latest upstream Rockbox master (25 commits) into the Shanling Q2 port. For this target that brings ATRAC3 codec fixes (gain compensation, MDCT window, output precision) and new recording-screen skin tags; existing themes and settings are unaffected. Install by unzipping rockbox.zip over .rockbox on the card; your config.cfg is kept.