refs #380, use CWorker instead of QConcurrent for threaded reader (bookings/VATSIM file)

src/blackmisc/worker.h

@@ -130,7 +130,7 @@ namespace BlackMisc
 
         //! Returns true if the task has finished.
         //! \threadsafe But don't rely on this condition remaining true for any length of time.
-        bool isFinished() const
+        virtual bool isFinished() const
         {
             QMutexLocker lock(&m_finishedMutex);
             return m_finished;