Make aud_input_ functions static members of InputPlugin.
Fix read_gain_from_tuple() if tuple has no Replay Gain info.
Add Tuple::get_replay_gain().
Fix build (modified audio.h, not audio.h.in, oops).
Fix build with --enable-valgrind.
Make playback functions asynchronous.
Fixes the UI hanging during fade-out of the last song with crossfade.
Tighter coupling with the playlist also fixes several race conditions.
Probably breaks FileWriter (again).
Add OutputPlugin::set_info().