public void Execute(object parameter)
            {
                ToDoList.Clear();
                List <PostIt> toDoPostits = PersistenceHandler.LoadTodoList();

                foreach (var postIt in toDoPostits)
                {
                    ToDoList.Add(postIt);
                }

                DoingList.Clear();
                List <PostIt> doingPostits = PersistenceHandler.LoadDoingList();

                foreach (var postIt in doingPostits)
                {
                    DoingList.Add(postIt);
                }

                DoneList.Clear();
                List <PostIt> donePostits = PersistenceHandler.loadDoneList();

                foreach (var postIt in donePostits)
                {
                    DoneList.Add(postIt);
                }
            }
 public void Execute(object parameter)
 {
     PersistenceHandler.Save(ToDoList, DoingList, DoneList);
 }