OpenCS: Render cell markers

There are still 2 problems with this branch, and I would like to hear what you think.

1) on windows at least, you need OGreOverlay.lib . I don't know how to add this to opencs CMAke.

2) I don't exactly know what to do regarding resource groups and Ogre. Right now, Ogre is not aware of resources/OpenCS so I had to manualy place the font Comic.ttf in some other places. Besides, are the resource groups name fixed?  See PagedWorldSpaceWidget line 267...