How to style GridControl's group summary items via the GroupColumnSummaryItemTemplate property

When group summary items are aligned by columns, it is necessary to use the TableView's GroupColumnSummaryItemTemplate property to style GridControl group summary items. The main idea of this approach is to define a custom ItemsControl inside the GroupColumnSummaryItemTemplate, bind its ItemsSource property to the collection of group summaries and show summary items as child ItemsControl elements. To customize summaries you can use ItemsControl properties, for instance, ItemTemplateSelector.