Start threads and check for inactive workers.

def _start(self, worker=None):
        """Start threads and check for inactive workers."""
        if worker:
            self._queue_workers.append(worker)

        if self._queue_workers and self._running_threads < self._max_threads:
            #print('Queue: {0} Running: {1} Workers: {2} '
            #       'Threads: {3}'.format(len(self._queue_workers),
            #                                 self._running_threads,
            #                                 len(self._workers),
            #                                 len(self._threads)))
            self._running_threads += 1
            worker = self._queue_workers.popleft()
            thread = QThread()
            if isinstance(worker, PythonWorker):
                worker.moveToThread(thread)
                worker.sig_finished.connect(thread.quit)
                thread.started.connect(worker._start)
                thread.start()
            elif isinstance(worker, ProcessWorker):
                thread.quit()
                worker._start()
            self._threads.append(thread)
        else:
            self._timer.start()

        if self._workers:
            for w in self._workers:
                if w.is_finished():
                    self._bag_collector.append(w)
                    self._workers.remove(w)

        if self._threads:
            for t in self._threads:
                if t.isFinished():
                    self._threads.remove(t)
                    self._running_threads -= 1

        if len(self._threads) == 0 and len(self._workers) == 0:
            self._timer.stop()
            self._timer_worker_delete.start()