title: PyQt : How to add a grid layout inside a QGroupBox in PyQt4
body: <p>I am trying to create an application window with PyQt4. I want to create a window with a frame and inside that frame some widgets such as labels and text editors.
  I created the frame as a QGroupBox to be able to put a title on it.
  I know that HBox and VBox seem to be the prefered layout when you deal with frames, however, I would like to manage the positionning of the  widgets inside my frame with a grid Layout, which I find easier to manage.
  So I tried this piece of code :</p>
  
  <pre><code>import sys
  from PyQt4 import QtGui, QtCore
  
  class Example(QtGui.QWidget):
  
      def __init__(self):
          super(Example, self).__init__()
  
          self.initUI()
  
  def initUI(self):      
  
      hbox = QtGui.QHBoxLayout()
      grid = QtGui.QGridLayout()
  
      #Definition des Tracing Parameters widgets
      WindowSize = QtGui.QLabel("Window size (m)")
      SampPts = QtGui.QLabel("Sampling points")
      WindowSizeEdit = QtGui.QLineEdit()
      SampPtsEdit = QtGui.QLineEdit()
      TracParamFrame = QtGui.QGroupBox(self)
      TracParamFrame.setTitle("Tracing Parameters")
      hbox.addLayout(grid)
  
      grid.addWidget(WindowSize,0,0)
      grid.addWidget(WindowSizeEdit,0,1)
      grid.addWidget(SampPts,1,0)
      grid.addWidget(SampPtsEdit,1,1)
  
      self.setLayout(hbox)
  
      self.setGeometry(300,300,350,300)
      self.show()
  
  
  def main():
  
      app = QtGui.QApplication(sys.argv)
      ex = Example()
      sys.exit(app.exec_())
  
  
  if __name__ == '__main__':
      main()
  </code></pre>
  
  <p>The main idea here was to create an hbox where I put the QGroupBox and then place a grid layout inside. 
  The problem is that in the application generated, the widgets are placed outside the frame, and in addition I get the error :</p>
  
  <p>QLayout: Attempting to add QLayout "" to Example "", which already has a layout
  QWidget::setLayout: Attempting to set QLayout "" on Example "", which already has a layout</p>