player: emit PLAYLIST event when stream tag changes

Commit b3e2635a introduced a regression: when a stream tag was
changed, the playlist version had to be updated.  This was done in
syncCurrentPlayerDecodeMetadata(), called by syncPlayerAndPlaylist().
After b3e2635a, this was not called anymore.  Fix this by emitting
PIPE_EVENT_PLAYLIST.

src/player_thread.c

@@ -116,6 +116,9 @@ static int player_wait_for_decoder(struct player *player)
 	player->queued = false;
 	player->decoder_starting = true;
 
+	/* call syncPlaylistWithQueue() in the main thread */
+	event_pipe_emit(PIPE_EVENT_PLAYLIST);
+
 	return 0;
 }
 
@@ -242,6 +245,10 @@ play_chunk(struct song *song, struct music_chunk *chunk,
 			if (old_tag != NULL)
 				tag_free(old_tag);
 
+			/* the main thread will update the playlist
+			   version when he receives this event */
+			event_pipe_emit(PIPE_EVENT_PLAYLIST);
+
 			/* notify all clients that the tag of the
 			   current song has changed */
 			idle_add(IDLE_PLAYER);