fix: add todo to SCThingsWithoutDiff and SCClasses

Closes #39

src/core/things/ToDo.ts

@@ -13,7 +13,7 @@
  * this program. If not, see <https://www.gnu.org/licenses/>.
  */
 import {SCThingWithCategoriesSpecificValues, SCThingWithCategoriesWithoutReferences} from '../base/ThingWithCategories';
-import {SCThingType} from '../Thing';
+import {SCThingMeta, SCThingType} from '../Thing';
 import {SCISO8601Date} from '../types/Time';
 
 /**
@@ -58,3 +58,9 @@ export enum SCToDoPriority {
   NORMAL = 2,
   HIGH = 5,
 }
+
+/**
+ * "To do" meta data
+ */
+export class SCToDoMeta extends SCThingMeta {
+}