OpenCS: Render cell markers

This is gugus's work (PR  #187).  All I did was merge in the master branch and made it compile with gcc on ubuntu.

Re-implemented using overlays.
Here an OverlaySystem is created for each scene widget. This is a problem, because OverlayManager (owned by OverlaySystem) is a singleton and will fail an assertion in its constructor if another OverlayManager has already been created. It looks like Ogre's overlay system isn't designed to be used independently with multiple scene managers.