Decouple command/OtherCommands.cxx from the "simple" database plugin.

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

Using a variable-size struct with embedded string is not worth the
trouble here.  There are not so many Directory objects.

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.

Explicit attribute initialization.

Replaces GLib's GError.

Let the database plugin do the match.