TreeModel: Make TreeModelVisitor useful

Functions that are meant to be overridden should be virtual.
Also add a convience level() function, seems to be a common need.

Change-Id: I09d452ab12f0ac29fb31de853057511418a47e7e
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/libs/utils/treemodel.cpp

@@ -777,10 +777,13 @@ void TreeItem::setModel(TreeModel *model)
 void TreeItem::walkTree(TreeItemVisitor *visitor)
 {
     if (visitor->preVisit(this)) {
+        ++visitor->m_level;
         visitor->visit(this);
         foreach (TreeItem *item, m_children)
             item->walkTree(visitor);
+        --visitor->m_level;
     }
+    visitor->postVisit(this);
 }
 
 void TreeItem::walkTree(std::function<void (TreeItem *)> f)

src/libs/utils/treemodel.h

@@ -49,11 +49,18 @@ class TreeModel;
 class QTCREATOR_UTILS_EXPORT TreeItemVisitor
 {
 public:
-    TreeItemVisitor() {}
+    TreeItemVisitor() : m_level(0) {}
     virtual ~TreeItemVisitor() {}
 
-    bool preVisit(TreeItem *) { return true; }
-    void visit(TreeItem *) {}
+    virtual bool preVisit(TreeItem *) { return true; }
+    virtual void visit(TreeItem *) {}
+    virtual void postVisit(TreeItem *) {}
+
+    int level() const { return m_level; }
+
+private:
+    friend class TreeItem;
+    int m_level;
 };
 
 class QTCREATOR_UTILS_EXPORT TreeItem