title: Translate standardButton in QDialogButtonBox
body: <p>i am using <em>pyqt</em> and <em>designer</em>. i have translated all the strings in my app with self.tr() + pylupdate4 and lrelease</p>
  
  <p>here is the snippet of code in my main():</p>
  
  <pre><code>app = QtGui.QApplication(sys.argv)
  app.setApplicationName('Mental Calculation')
  
  # initialize locale and load translation files if available
  locale = QtCore.QLocale()
  LOCALENAME = str(locale.system().name())
  translator = QtCore.QTranslator()
  translator.load("mentalcalculation_%s" % LOCALENAME)
  app.installTranslator(translator)
  </code></pre>
  
  <p>I use a <em>QDialogButtonBox</em> in a <em>QDialog</em> with a <em>QtGui.QDialogButtonBox.Cancel</em> and a <em>QtGui.QDialogButtonBox.Ok</em></p>
  
  <p>and the strings in these buttons are not translated. because <em>pylupdate4</em> does pick any string for them. </p>
  
  <p>Have I missed a configuration step in my app so that they are translated ? 
  I don't understand how the string for standard buttons of <em>QDialogButtonBox</em> are supposed to be translated and can't found doc about that.</p>