scene: Add CNode & CEntityNode stubs
graphics-irr: Remove CConversionTable

This dirty hack was added for the sake of keeping the record of my ideas.
It served as a temporary mean to keep two different scene graphs in sync.
Now I devised a replacement, which will utilize Irrlicht's own mechanisms.
The idea is to create a dummy irr::scene::ISceneNode, which will override
getAbsoluteTransformation(). Instead of returning it's own absolute
transform, it will walk up the hrengin scene graph, beginning at it's
parent node and calculate absolute transform. Probably I will try to
optimize it at some point. Or maybe not, since I am planning on making my
own render system.