FileManagerListJob: simplify waiting for worker thread

Wait for the QFuture returned by QtConcurrent::run() to finish instead
of custom QSemaphore locking.

Thread cancellation and waiting for finished in ~FileManagerListJob()
still works as expected. Tested it by inserting `QThread::sleep(5);`
right after the `const auto entries =` line in the worker thread lambda
and inserting several tracing lines like this:
    qCritical() << "destroying" << this << QTime::currentTime().second()
                << QTime::currentTime().msec();