TaurusAttrListComboBox fails when device is not exported

[reszelaz]

This issue appears after integration of #1002. One can reproduce it with python3 -m taurus.qt.qtgui.input.tauruscombobox after prior changing the main funtion to call _taurusAttrListTest instead of _taurusValueComboboxTest. The clue is to have the TangoTest server down. This affects the taurus gui macrogui.

[cmft]

It seems a side effect of calling Timer::stop() synchronous. This feature was added in #1002 (to solve circular reference in py3)

    def stop(self, sync=False):
        """ Stop Timer Object """
        self.debug("Timer::stop()")
        self.__lock.acquire()
        self.__loop = False
        self.__lock.release()
        if sync and self.__thread is not None:
            self.__thread.join()