title: Find the center position of a QGraphicsScene?
body: <p>I have tried to use  the following code to add a line to my scene:</p>
  
  <pre><code>    line = new QGraphicsLineItem(200,55,200,55);
      mscene.addItem(ruler);
  </code></pre>
  
  <p>But it seems the coordinate of the QGraphicsLineItem starts at the left corner.
  I don't want to alter the origin any way. I would like to just retrieve the center position's coordinate of the qgraphicsscene/qgraphicsview. 
  What function should I use?</p>