Added central user credentials management

lib/model/accountModel.dart

@@ -1,17 +1,17 @@
 import 'package:flutter/cupertino.dart';
 
 class AccountModel extends ChangeNotifier {
-  bool _isLoggedIn = false;
+  AccountModelState _accountState = AccountModelState.undefined;
+  AccountModelState get state => _accountState;
 
-  bool get isLoggedIn => _isLoggedIn;
-
-  void logout() {
-    _isLoggedIn = false;
+  void setState(AccountModelState state) {
+    _accountState = state;
     notifyListeners();
   }
+}
 
-  void login() {
-    _isLoggedIn = true;
-    notifyListeners();
-  }
+enum AccountModelState {
+  undefined,
+  loggedIn,
+  loggedOut,
 }