TaskTree: Implement simple progress reporting

The progressMaximum is the number of tasks in tree.
The progressValueChanged signal is emitted whenever task is done
(advance by one) or when some number of tasks were skipped or stopped
due to workflow policy or dynamic setup group handler.

Change-Id: I403059a6466ae0534ef647c3c1c61c0318f10325
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/tasktree.h

@@ -246,10 +246,14 @@ public:
     void stop();
     bool isRunning() const;
     int taskCount() const;
+    int progressMaximum() const { return taskCount(); }
+    int progressValue() const; // all finished / skipped / stopped tasks, groups itself excluded
 
 signals:
+    void started();
     void done();
     void errorOccurred();
+    void progressValueChanged(int value); // updated whenever task finished / skipped / stopped
 
 private:
     TaskTreePrivate *d;