event_pipe: added "TAG" event

The "TAG" event is emitted by the player thread when the current
song's tag has changed.  Split this event from "PLAYLIST" and make it
a separate callback, which is more efficient.

src/event_pipe.h

@@ -36,6 +36,9 @@ enum pipe_event {
 	/** must call syncPlayerAndPlaylist() */
 	PIPE_EVENT_PLAYLIST,
 
+	/** the current song's tag has changed */
+	PIPE_EVENT_TAG,
+
 	/** SIGHUP received: reload configuration, roll log file */
 	PIPE_EVENT_RELOAD,