Automate the life cycle, making the code safer.

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 pass a GError** to g_key_file_get_X().  We don't need to dispose
something we didn't request in the first place.

The MPD core takes care for checking the scheme.

Merge some duplicate code.

Replaces GLib's g_str_has_prefix().

Remove the forward_list::reverse() call.  It was not necessary,
because pls_parser() already reads the playlist in reverse order.