title: Widget's "destroyed" signal is not fired (PyQT)
body: <p>I have a widget which would have to do some manual cleanup after it's destroyed (stop some threads). However for some reason the "destroyed" signal of the widget is not firing. I have made this small example that demonstrates the problem.</p>
  
  <pre><code>import sys
  from PyQt4 import QtGui
  
  class MyWidget(QtGui.QWidget):
      def __init__(self, parent):
          super(MyWidget, self).__init__(parent)
  
          def doSomeDestruction():
              print('Hello World!')
  
          self.destroyed.connect(doSomeDestruction)
  
  
  class MyWindow(QtGui.QMainWindow):
      def __init__(self):
          super(MyWindow, self).__init__()
  
          self.widget = MyWidget(self)
  
  app = QtGui.QApplication(sys.argv)
  window = MyWindow()
  window.show()
  ret = app.exec_()
  sys.exit(ret)
  </code></pre>
  
  <p>I expected it to print "Hello World!" when the main window is closed. However, it doesn't print anything.</p>