Added automatic refreshing of Notification token

lib/app.dart

@@ -47,7 +47,9 @@ class _AppState extends State<App> {
     });
 
     if(Provider.of<SettingsProvider>(context, listen: false).val().notificationSettings.enabled) {
-      NotifyUpdater.registerToServer();
+      FirebaseMessaging.instance.onTokenRefresh.listen((event) {
+        NotifyUpdater.registerToServer();
+      });
     }
 
     FirebaseMessaging.onMessage.listen((message) => NotificationController.onForegroundMessageHandler(message, context));