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.
> There is always harm in a memory leak.

Agree with you in a general sense.  In this particular situation, having an instance of OverlaySystem is similar to the workaround we apply with a hidden instance of 1x1 Ogre Window (which takes up memory and only gets destroyed when OpenCS exists).  OverlaySystem does not get lost and we only ever have one instance (which we keep as a static member to SceneWidget).  It's lifetime is determined by if & when a scenewidget is constructed and when opencs exits.

This is why I consider that the "harm" is minimal, if any.  Having said that, I'm open to suggestions if there are better ways to deal with this.  Static variables are not ideal, either, anyway.