TaskTree: Add default c'tor and setupRoot() method

Make it possible to setup root later, after TaskTree
was already constructed.

Change-Id: I59e8e6a12102f9b8ed0f40357ab12e47d67572c2
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/utils/tasktree.h

@@ -232,9 +232,12 @@ class QTCREATOR_UTILS_EXPORT TaskTree : public QObject
     Q_OBJECT
 
 public:
+    TaskTree();
     TaskTree(const Tasking::Group &root);
     ~TaskTree();
 
+    void setupRoot(const Tasking::Group &root);
+
     void start();
     void stop();
     bool isRunning() const;