TreeModel: Initialize root item with proper type in LeveledTreeModel

Current usage is to use setRootItem() manually when needed, but
since the type is known in a LeveledTreeModel this can be automatic.

Change-Id: I511acdf9b9b3f4b67cf87d07b5446baa329a6a77
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/utils/treemodel.h

@@ -250,7 +250,7 @@ template <class RootItem,
 class LeveledTreeModel : public TreeModel
 {
 public:
-    explicit LeveledTreeModel(QObject *parent = 0) : TreeModel(parent) {}
+    explicit LeveledTreeModel(QObject *parent = 0) : TreeModel(new RootItem, parent) {}
     explicit LeveledTreeModel(RootItem *root, QObject *parent = 0) : TreeModel(root, parent) {}
 
     template <class Predicate>