save method
Saves data.json into the editor's working folder (FileManager.tempFolder, next to layers/ and
sounds/; 1.1.0 wrote it one level up, where load after an unzip never found it).
Implementation
Future<void> save() async {
final dir = await FileManager.tempFolder;
await Directory(dir).create(recursive: true);
await File('$dir/data.json').writeAsString(jsonEncode(toJson()));
}