title: Went from QGraphicsView to QMainWindow now keyPressEvent doesn't see arrow keys
body: <p>I'm trying to learn about the event handling in PyQt4 and stumbled upon a case where
  <code>keyPressEvent</code> sees all keys except the arrow keys after I changed the class definition.</p>
  
  <p>From seeing all keys:</p>
  
  <pre><code>class MaskWindow(QtGui.QGraphicsView):
      def __init__(self):
          QtGui.QGraphicsView.__init__(self)
  
          self.deskTop = QtGui.QDesktopWidget()
          self.scene = QtGui.QGraphicsScene(self)
  </code></pre>
  
  <p>To can't see the arrow keys, page-down and up, but others work e.g. <kbd>Tab</kbd>, <kbd>Shift</kbd>, <kbd>a</kbd>-<kbd>z</kbd>:</p>
  
  <pre><code>class MaskWindow(QtGui.QMainWindow):
      def __init__(self):
          QtGui.QMainWindow.__init__(self, None, QtCore.Qt.WindowStaysOnTopHint|
                                     QtCore.Qt.FramelessWindowHint)            
  
          self.deskTop = QtGui.QDesktopWidget()
          self.view = QtGui.QGraphicsView(self)
          self.scene = QtGui.QGraphicsScene(self)
  </code></pre>
  
  <p>Here's the key handler where the arrow key doesn't trap anymore:</p>
  
  <pre><code>def keyPressEvent(self, event):
      key = event.key()
      if key == QtCore.Qt.UpArrow
          self.diameter += 1
      if key == QtCore.Qt.DownArrow:
          self.diameter -= 1
  </code></pre>
  
  <p>What did I miss with the class change?</p>