Avoid duplicate AllocatedPath::FromUTF8() invocations in two callers.

Use Storage::MapUTF8() internally, don't use global variables.

Reduce bloat.

Prepare for UPnP songs that retain there database identity.

Suppresses gcc warnings.

Remove unnecessary explicit code.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Make the DetachedSong(Song) conversion constructor private.  Everybody
should use map_song_detach() which will take over more
responsibilities soon.

Don't create an intermediate Song instance when all we want is a
DetachedSong.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.