XBEL stream example: use constructor initialization

The treeWidget member of MainWindow could be initialized before the
body of the constructor, enabling it to be a *const variable.

Pick-to: 6.6 6.5
Task-number: QTBUG-111228
Change-Id: If4a3b04729bc7fa5859ca88183eec376f6992455
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

examples/corelib/serialization/streambookmarks/mainwindow.cpp

@@ -24,8 +24,8 @@ using namespace Qt::StringLiterals;
 
 //! [0]
 MainWindow::MainWindow()
+    : treeWidget(new QTreeWidget)
 {
-    treeWidget = new QTreeWidget;
     treeWidget->header()->setSectionResizeMode(QHeaderView::Stretch);
     treeWidget->setHeaderLabels(QStringList{ tr("Title"), tr("Location") });
 #if QT_CONFIG(clipboard) && QT_CONFIG(contextmenu)

examples/corelib/serialization/streambookmarks/mainwindow.h

@@ -28,7 +28,7 @@ public slots:
 private:
     void createMenus();
 
-    QTreeWidget *treeWidget;
+    QTreeWidget *const treeWidget;
 };
 //! [0]