title: QDialog not opening from Main window (pyQt)
body: <p>I'm trying to launch a dialog by clicking a button in the main window: Here's the (qtdesigner generated) code which I modified just to test it .. I've set the showDial function to show the dial when the button is clicked. But it doesn't work :</p>
  
  <pre><code>from PyQt4 import QtCore, QtGui
  
  try:
      _fromUtf8 = QtCore.QString.fromUtf8
  except AttributeError:
      _fromUtf8 = lambda s: s
  
  
  
  class Ui_Dialog(object):
      def setupUi(self, Dialog):
          Dialog.setObjectName(_fromUtf8("Dialog"))
          Dialog.setWindowModality(QtCore.Qt.WindowModal)
          Dialog.resize(400, 300)
          Dialog.setWindowTitle(QtGui.QApplication.translate("Dialog", "Dialog", None,   QtGui.QApplication.UnicodeUTF8))
  
          self.retranslateUi(Dialog)
          QtCore.QMetaObject.connectSlotsByName(Dialog)
  
      def retranslateUi(self, Dialog):
          pass
  
  class Ui_MainWindow(object):
      def setupUi(self, MainWindow):
          MainWindow.setObjectName(_fromUtf8("MainWindow"))
          MainWindow.resize(309, 148)
          MainWindow.setWindowTitle(QtGui.QApplication.translate("MainWindow", "MainWindow", None, QtGui.QApplication.UnicodeUTF8))
          self.centralwidget = QtGui.QWidget(MainWindow)
          self.centralwidget.setObjectName(_fromUtf8("centralwidget"))
          self.pushButton = QtGui.QPushButton(self.centralwidget)
          self.pushButton.setGeometry(QtCore.QRect(50, 30, 191, 71))
          self.pushButton.setText(QtGui.QApplication.translate("MainWindow", "Open Dialog", None, QtGui.QApplication.UnicodeUTF8))
          self.pushButton.setObjectName(_fromUtf8("pushButton"))
          MainWindow.setCentralWidget(self.centralwidget)
  
          self.retranslateUi(MainWindow)
  
          QtCore.QObject.connect(self.pushButton, QtCore.SIGNAL(_fromUtf8("clicked(QAbstractButton*)")), self.showDial)
  
          QtCore.QMetaObject.connectSlotsByName(MainWindow)
  
  
  
      def retranslateUi(self, MainWindow):
          pass
  
      def showDial(self):
        Dialog = QtGui.QDialog()
        u = Ui_Dialog()
        u.setupUi(Dialog)
  
        Dialog.exec_()
  
  
  if __name__ == "__main__":
      import sys
      app = QtGui.QApplication(sys.argv)
      MainWindow = QtGui.QMainWindow()
      ui = Ui_MainWindow()
      ui.setupUi(MainWindow)
      MainWindow.show()
      sys.exit(app.exec_())
  </code></pre>