title: MousePressEvent, position offset in QGraphicsView
body: <p>I've some difficulties with <code>QGraphicsView</code> and <code>QGraphicsScene</code>. 
  When I zoom/unzoom in the scene and create items with mousePressEvent, I have an offset in the position. How can this be avoided?</p>
  
  <p><code>event.pos()</code> seems to be the problem.. </p>
  
  <pre class="lang-py prettyprint-override"><code>from PyQt4 import QtCore, QtGui
  
  class graphicsItem (QtGui.QGraphicsItem):
      def __init__ (self):
          super(graphicsItem, self).__init__()
          self.rectF = QtCore.QRectF(0,0,10,10)
      def boundingRect (self):
          return self.rectF
      def paint (self, painter=None, style=None, widget=None):
          painter.fillRect(self.rectF, QtCore.Qt.red)
  
  class graphicsScene (QtGui.QGraphicsScene):
      def __init__ (self, parent=None):
          super (graphicsScene, self).__init__ (parent)
  
  class graphicsView (QtGui.QGraphicsView):
      def __init__ (self, parent = None):
          super (graphicsView, self).__init__ (parent)
          self.parent = parent
      def mousePressEvent(self, event):
          super (graphicsView, self).mousePressEvent(event)
          item = graphicsItem()
          position = QtCore.QPointF(event.pos()) - item.rectF.center()
          item.setPos(position.x() , position.y())
          self.parent.scene.addItem(item)
      def wheelEvent (self, event):
          super (graphicsView, self).wheelEvent(event)
          factor = 1.2
          if event.delta() &lt; 0 :
              factor = 1.0 / factor
          self.scale(factor, factor)
  
  class window (QtGui.QMainWindow):
      def __init__ (self, parent = None ) :
          super (window, self).__init__(parent)
          self.width = 800
          self.height = 600
  
          self.resize(self.width,self.height)
          self.mainLayout = QtGui.QVBoxLayout(self)
  
          self.view = graphicsView(self)
          self.scene = graphicsScene(self)
          self.view.setScene (self.scene)
  
          factor = 1
          self.scene.setSceneRect(0, 0, self.width * factor, self.height * factor)
          self.view.setMinimumSize(self.width, self.height)
  
          self.mainLayout.addWidget(self.view)
  
      def show (self):
          super (window, self).show()  
  </code></pre>