Loading assets with assimp.

ATTENTION: this commit breaks the entire project. It's an unfinished
work and is being commited just for backup purposes. Person from the
future: *never* revert to this commit! You've been warned.

What I am doing: modifying everything to load assets with the Open
Assets Import Library (assimp).

I just finished setting up camera and light sources as well as
instantianting GraphicsItems from the meshes provided by the aiScene.

Next steps include fixing the build, remove unnecessary code from all
classes and the more important thing: RENDERING.