Utils: Make remaining TreeModel iterating functions 'const'

This follows the rest of similar functions. It is arguably abusing the
C++ misfeature that 'const' only applies to the top level, but
avoids re-typing the same implementations stanzas for otherwise needed
const/non-const overloads.

Change-Id: If90fc879b406fd94714995ff4c42a4ebe3c69faa
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/utils/treemodel.h

@@ -91,13 +91,13 @@ public:
 
     // Levels are 1-based: Child at Level 1 is an immediate child.
     template <class T, typename Predicate>
-    void forFirstLevelChildren(Predicate pred) {
+    void forFirstLevelChildren(Predicate pred) const {
         foreach (TreeItem *item, m_children)
             pred(static_cast<T>(item));
     }
 
     template <class T, typename Predicate>
-    void forSecondLevelChildren(Predicate pred) {
+    void forSecondLevelChildren(Predicate pred) const {
         foreach (TreeItem *item1, m_children)
             foreach (TreeItem *item2, item1->m_children)
                 pred(static_cast<T>(item2));