Fix Editable Tree Model example model rowCount and model instance leak

The rowCount method has been fixed to correctly pass the "Common error
test #3: the second column should NOT have the same children".

It also fixes the model being leaked.

Fixes: QTBUG-92178
Pick-to: 5.15
Change-Id: If92973e6f83ea9a2715bd335269f6e50d80f52c6
Reviewed-by: Luca Beldi <v.ronin@yahoo.it>
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

examples/widgets/itemviews/editabletreemodel/mainwindow.cpp

@@ -62,7 +62,7 @@ MainWindow::MainWindow(QWidget *parent)
 
     QFile file(":/default.txt");
     file.open(QIODevice::ReadOnly);
-    TreeModel *model = new TreeModel(headers, file.readAll());
+    TreeModel *model = new TreeModel(headers, file.readAll(), this);
     file.close();
 
     view->setModel(model);

examples/widgets/itemviews/editabletreemodel/treemodel.cpp

@@ -212,6 +212,9 @@ bool TreeModel::removeRows(int position, int rows, const QModelIndex &parent)
 //! [8]
 int TreeModel::rowCount(const QModelIndex &parent) const
 {
+    if (parent.isValid() && parent.column() > 0)
+        return 0;
+
     const TreeItem *parentItem = getItem(parent);
 
     return parentItem ? parentItem->childCount() : 0;