MetaFile::Album: support for embedded album cover reading & writing

FEATURES:
* Support for embedded album covers in non-collection tracks and
in USB Mass Storage collection.

This could potentially slow down performace of album loading, but my
tests show that cover art loading takes 2 - 10ms, which is really okay.
Also, the fact that track has embedded cover is cached, so there is no
penalty for tracks wihout cover.

DIGEST: Feature