Ditch external module, rename toplevel to opengl

* Remove the external column renderer module since it will probably be
built into the framework itself to avoid back-and-forth communication
slow-down.
* Rename Toplevel to OpenGL to represent the fact that what we are
controlling is an OpenGL context and not just a standard window. This
module will be the primary way of talking directly to OpenGL, binding
textures and sending draw commands and whatnot.
* Add guards to the headers.