#8 Added marker to indicate read status in talk chats

lib/api/requestCache.dart

@@ -1,10 +1,11 @@
 import 'dart:convert';
 
 import 'package:localstore/localstore.dart';
+import 'package:marianum_mobile/api/apiResponse.dart';
 
 import 'webuntis/webuntisError.dart';
 
-abstract class RequestCache<T> {
+abstract class RequestCache<T extends ApiResponse?> {
   static const int cacheNothing = 0;
   static const int cacheMinute = 60;
   static const int cacheHour = 60 * 60;