Fixes aborted "load" commands due to SongLoader failure.

Now that Song::GetDuration() is used (which considers start_ms and
end_ms), we don't need to override the Tag's duration value.

Playback with some decoder plugins was broken because the Queue's
DetachedSong instances did not have the "real" URI (the mapped path).

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Reduce bloat.

This has been verified already in the caller.

Allows applying map_to_relative_path() with base_uri.

No caller passes a database song to this function, but if he does,
this check doesn't make sense.

This didn't make sense.

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

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.

Don't use IgnoreError() when there's an overload that does not try to
give us one.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.