TOFIX [ENGINE]: refactor collision system; remove CollisionTestDemo

[steaklive]

move collision tests from a hardcoded cpp level into a loadable system (i.e., attach a component to RenderinObject).
this will take a lot of work, which is why not a priority atm

[steaklive]

I decided to remove all collision-related code from the project (bafad73). That code was written/added to the engine years ago as a part of my university course assignment, however, collision checks, CPU spatial hierarchies (and OBB support) do not really fit into the concept of the project (rendering engine), which is why they only pollute code. By the moment of writing this comment, I could not find any other use for those systems in my engine. Maybe in the future, I will write new, better-coded systems for spatial structures, like an octree or BVH.