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.cxx

@@ -97,6 +97,12 @@ Partition::TagModified()
 		playlist.TagModified(std::move(*song));
 }
 
+void
+Partition::TagModified(const char *uri, const Tag &tag) noexcept
+{
+	playlist.TagModified(uri, tag);
+}
+
 void
 Partition::SyncWithPlayer()
 {