http://bugs.musicpd.org/view.php?id=4470

Manipulates the playback range of a queued song.

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

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.

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

Fixes purging deleted songs from the queue after update.  Fixes
regression from commit e96779de.

Fix for a major memory leak.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.