TreeModel: Fix model back pointer propagation when adding items

Only the top level entry was handled so far, but we might add
items with subitems.

Change-Id: I55084a1f595f4982bd37168e71b144b17eee63b1
Reviewed-by: hjk <hjk@theqtcompany.com>

src/libs/utils/treemodel.cpp

@@ -695,7 +695,7 @@ void TreeItem::insertChild(int pos, TreeItem *item)
         QModelIndex idx = index();
         m_model->beginInsertRows(idx, pos, pos);
         item->m_parent = this;
-        item->m_model = m_model;
+        item->propagateModel(m_model);
         m_children.insert(m_children.begin() + pos, item);
         m_model->endInsertRows();
     } else {