Song: add method ReplaceTag()

src/Song.cxx

@@ -109,6 +109,14 @@ Song::Free()
 	g_free(this);
 }
 
+void
+Song::ReplaceTag(Tag &&_tag)
+{
+	if (tag == nullptr)
+		tag = new Tag();
+	*tag = std::move(_tag);
+}
+
 gcc_pure
 static inline bool
 directory_equals(const Directory &a, const Directory &b)

src/Song.hxx

@@ -126,6 +126,8 @@ struct Song {
 		return parent == &detached_root;
 	}
 
+	void ReplaceTag(Tag &&tag);
+
 	bool UpdateFile();
 	bool UpdateFileInArchive();