New event for Collection metadata updfate

lib/events/collection_meta_event.dart

@@ -0,0 +1,16 @@
+import 'package:photos/events/event.dart';
+
+class CollectionMetaEvent extends Event {
+  final int id;
+  final CollectionMetaEventType type;
+
+  CollectionMetaEvent(this.id, this.type);
+}
+
+enum CollectionMetaEventType {
+  created,
+  deleted,
+  archived,
+  sortChanged,
+  thumbnailChanged,
+}