Added central user credentials management

lib/view/pages/timetable/timetable.dart

@@ -2,7 +2,6 @@
 import 'dart:developer';
 
 import 'package:flutter/material.dart';
-import 'package:marianum_mobile/widget/loadingSpinner.dart';
 import 'package:provider/provider.dart';
 import 'package:syncfusion_flutter_calendar/calendar.dart';
 
@@ -11,7 +10,8 @@ import '../../../api/webuntis/queries/getRooms/getRoomsResponse.dart';
 import '../../../api/webuntis/queries/getSubjects/getSubjectsResponse.dart';
 import '../../../model/timetable/timetableProps.dart';
 import '../../../storage/base/settingsProvider.dart';
-import '../../../widget/errorView.dart';
+import '../../../widget/loadingSpinner.dart';
+import '../../../widget/placeholderView.dart';
 import 'appointmenetComponent.dart';
 import 'appointmentDetails.dart';
 import 'timeRegionComponent.dart';
@@ -68,7 +68,7 @@ class _TimetableState extends State<Timetable> {
           GetHolidaysResponse holidays = value.getHolidaysResponse;
 
           if(value.hasError) {
-            return ErrorView(
+            return PlaceholderView(
               icon: Icons.calendar_month,
               text: "Webuntis error: ${value.error.toString()}",
             );