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/queue/Playlist.cxx

@@ -43,6 +43,24 @@ playlist::TagModified(DetachedSong &&song)
 	OnModified();
 }
 
+void
+playlist::TagModified(const char *uri, const Tag &tag) noexcept
+{
+	bool modified = false;
+
+	for (unsigned i = 0; i < queue.length; ++i) {
+		auto &song = *queue.items[i].song;
+		if (song.IsURI(uri)) {
+			song.SetTag(tag);
+			queue.ModifyAtPosition(i);
+			modified = true;
+		}
+	}
+
+	if (modified)
+		OnModified();
+}
+
 inline void
 playlist::QueueSongOrder(PlayerControl &pc, unsigned order)
 

src/queue/Playlist.hxx

@@ -23,6 +23,7 @@
 #include "queue/Queue.hxx"
 
 enum TagType : uint8_t;
+struct Tag;
 struct PlayerControl;
 class DetachedSong;
 class Database;
@@ -192,6 +193,7 @@ public:
 	 * the song matches.
 	 */
 	void TagModified(DetachedSong &&song);
+	void TagModified(const char *uri, const Tag &tag) noexcept;
 
 #ifdef ENABLE_DATABASE
 	/**