Doc: QIdentityProxyModel::itemData() should be reimplemented too

This is a "regression" (voluntary, known) from commit
c27d2a57a441f9a1ce760e71635bd4c96882249d which changed
QAbstractProxyModel::itemData() to call the source model's itemData(),
so our data() reimplementation is no longer used.

Of course this only matters if itemData() is actually called, which
isn't the case in Qt itself. People will likely skip this if they don't
care - but if itemData() is used in the project, then this shows how
to reimplement it correctly.

Change-Id: I3acea16c05d30d7526bac32fd6cce42b5ad4b617
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit abc739600447af06befaca8ae616dd607b99b184)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit bbd868ff682c991bbd6cb88a9cf363e04871472b)

src/corelib/doc/snippets/code/src_gui_itemviews_qidentityproxymodel.cpp

@@ -17,10 +17,16 @@ class DateFormatProxyModel : public QIdentityProxyModel
       return QIdentityProxyModel::data(index, role);
 
     const QDateTime dateTime = sourceModel()->data(SourceClass::DateRole).toDateTime();
-
     return dateTime.toString(m_formatString);
   }
 
+  QMap<int, QVariant> itemData(const QModelIndex &proxyIndex) const override
+  {
+      QMap<int, QVariant> map = QIdentityProxyModel::itemData(proxyIndex);
+      map[Qt::DisplayRole] = data(proxyIndex);
+      return map;
+  }
+
 private:
   QString m_formatString;
 };