OpenGL Basic Grid
Adds a basic grid to the OpenGL viewport. I intend to add support for a rotated viewport and translations, however this has turned out to be a bit more complicated that I'd initially anticipated. Therefore I'm going to work on the logic for that on a different branch anyway so we could already merge this so we at least have a basic grid. And that would also make it possible to implement an on/off global setting already.
Part of #74 (closed)