Table View Example: Sort indicator for the custom header delegate

The Delegates tab in the example shows how to use custom types as
TableView header, row, and item. The UI has a checkbox to toggle
a sort indicator for the header - this change implements the
indicator for the custom header delegate, and adjusts the header
text accordingly.

Task-number: QTBUG-41396
Change-Id: Iab917b87b9b91c3dcc468f87aa14ec9293afd5f6
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>