Playlist: copy stream tags from the PlayerThread

Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779d).

src/Partition.hxx

@@ -167,8 +167,8 @@ struct Partition {
 	}
 
 	/**
-	 * A tag in the play queue has been modified.  Propagate the
-	 * change to all subsystems.
+	 * A tag in the play queue has been modified by the player
+	 * thread.  Propagate the change to all subsystems.
 	 */
 	void TagModified();