TaskTree: Add taskCount() getter

Will be helpful when implementing progressMaximum.

Change-Id: I9204b53425054a01c96f3b926b595f7888f81e3a
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/tasktree.h

@@ -245,6 +245,7 @@ public:
     void start();
     void stop();
     bool isRunning() const;
+    int taskCount() const;
 
 signals:
     void done();