private void addStageButton_Click(object sender, RoutedEventArgs e)
        {
            MemorizeStageWindow wnd = new MemorizeStageWindow(null);

            if (wnd.ShowDialog().Value)
            {
                this.memorizeEntry.Stages.Add(wnd.Stage);
                this.isChanged = true;
            }
        }
        private void editStageButton_Click(object sender, RoutedEventArgs e)
        {
            MemorizeStage       stage = this.stageListBox.SelectedItem as MemorizeStage;
            MemorizeStageWindow wnd   = new MemorizeStageWindow(stage);

            if (wnd.ShowDialog().Value)
            {
                this.memorizeEntry.Stages.Insert(this.stageListBox.SelectedIndex, wnd.Stage);
                this.memorizeEntry.Stages.Remove(stage);
                this.isChanged = true;
            }
        }