How to write a simple Python Qt dialog on ROS Service?

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I want to write a ROS Service that receives a Question and then shows a Qt Pop-up Dialog with the Question and a yes or no Button. The service should return the Answer as a Boolean. I have it with wx but since it is depreciated in ROS, I want to use Qt now. This is how it looked like with wx:

    rospy.init_node('dialog_server')
    s = rospy.Service('dialog', Dialog, self.handle_dialog)
    rospy.spin()
    def handle_dialog(self, req):
        ex = wx.App()
        dial = wx.MessageDialog(None, req.message, 'Question',
                        wx.YES_NO | wx.ICON_QUESTION)
        ret = dial.ShowModal()
        if ret == wx.ID_YES:
                answer = True
        else:
                answer = False
        return DialogResponse(answer)

I tried simply replacing wx with the equivalent qt (like wx.MessageDialog -> QtGui.QMessageBox.question) but it's not working because ros is another Thread. Error: "It is not safe to use pixmaps outside the GUI thread". I am not familiar with Qt, rqt python_qt_binding and plug-ins at all.

What do I need to achieve this? Do I have to use Slots and Signals between GUI and ROS, or even write it as a plug-in? How should it look like?