Make id3v2 plugin extract replaygain tags

Fixes #1636
OTHER: Add Simonas Kazlauskas to AUTHORS.
OTHER: First try actual int when getting int coll attr.
BUG(1755): Add support for 'seed' to Order coll type:random.
OTHER: Simplify idlist serialization as type is restricted.
OTHER: Simplify operands serialization as type is restricted.
OTHER: Refactor serializing collection operands.
OTHER: Remove unnecessary _valid() check when serializing lists/dicts.
OTHER: list/dict size is accessed in constant time.
BUG(2480): Replace int32 with int64 on the wire and in xmmstypes.