TreeModel: Introduce a TreeItem::indexInParent() convenience function

To shorten user code and hide the ugly const_cast.

Change-Id: I798bd105932004ea17cb95b243fc38ccfcda0a2c
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/utils/treemodel.h

@@ -65,6 +65,7 @@ public:
     int level() const;
 
     int childCount() const { return m_children.size(); }
+    int indexInParent() const;
     TreeItem *childAt(int index) const;
     QVector<TreeItem *> children() const { return m_children; }
     QModelIndex index() const;