For std::runtime_error.  See http://bugs.musicpd.org/view.php?id=4509

For free().

Don't depend on the "simple" database plugin.  This fixes an assertion
failure / crash and allows using stickers with other plugins.

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

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.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

sticker_delete_value() deletes only one value in a sticker, while
the old function sticker_delete() deletes all values.

This is a wrapper for sticker_find(), which looks up the song object.

sticker_list_values() is only used internally in sticker.c.  Remove
sticker_song_list_values() completely, it is superseded by
sticker_song_get().