Start the task.

        This is:
            * not threadsave
            * assumed to be called in the gtk mainloop

def start(self, *args, **kwargs):
        """Start the task.

        This is:
            * not threadsave
            * assumed to be called in the gtk mainloop
        """
        args = (self.counter,) + args
        thread = threading.Thread(
                target=self._work_callback,
                args=args, kwargs=kwargs
                )
        thread.setDaemon(self.daemon)
        thread.start()