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.
True.  However: Ogre::OverlaySystem sets of a couple of singletons (OverlayManager and FontManager) then re-implements renderQueueStarted().  I'm not sure if there is any real harm of keeping it around till OpenCS exits.

What is more problematic is how to delete it.  The pointer needs to be passed to editor.cpp somehow (create a singleton for the purpose?) and delete it in its destructor before Ogre::Root is destroyed.  Actually Editor doesn't seem to have a destructor to start with...

If I create OverlaySystem in editor.cpp, then I'll need a way of passing that to SceneWidget so that it can be added to its scene manager instance's render queue.  Maybe using a singleton.

Having a static member to hold OverlaySystem is a poor man's singleton, anyway.