title: Moving a QGraphicsItem around a central point in PyQt4
body: <p>I'm using Python 2.7 and PyQt4. I am trying to have a half-circle object that is a QGraphicsItem. I want to be able to move it using the mouse, by clicking and dragging. I can create the object and move it around with the mouse by setting the flag ItemIsMovable. Now the half-circle moves around freely but I want it to move just around the fixed central point. It is difficult to describe, but it should be something similar to a dial. How can I accomplish this?</p>