Update modified items in generic wxDataViewCtrl immediately

Recent optimizations avoiding resort on item change (see
https://github.com/wxWidgets/wxWidgets/pull/642) have also optimized
away refreshing the modified item, which was done implicitly, as a side
effect of resorting, before, so the changes were not reflected on
display immediately any longer.

Fix this by simply refreshing the item explicitly and also add a way to
test for the correct behaviour in the sample.

src/generic/datavgen.cpp

@@ -2959,6 +2959,9 @@ bool wxDataViewMainWindow::DoItemChanged(const wxDataViewItem & item, int view_c
         GetOwner()->InvalidateColBestWidth(view_column);
     }
 
+    // Update the displayed value(s).
+    RefreshRow(GetRowByItem(item));
+
     // Send event
     wxDataViewEvent le(wxEVT_DATAVIEW_ITEM_VALUE_CHANGED, m_owner, column, item);
     m_owner->ProcessWindowEvent(le);