RemoteTagCache: new glue class for integrating RemoteTagScanner

This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

src/Partition.hxx

@@ -226,6 +226,12 @@ struct Partition final : QueueListener, PlayerListener, MixerListener {
 	 */
 	void TagModified();
 
+	/**
+	 * The tag of the given song has been modified.  Propagate the
+	 * change to all instances of this song in the queue.
+	 */
+	void TagModified(const char *uri, const Tag &tag) noexcept;
+
 	/**
 	 * Synchronize the player with the play queue.
 	 */