Migrate to SDL 2

Ok, I know this comes a bit unexpected and all... but yes, this branch holds a complete migration to SDL 2!
I can think of a few reasons why we should migrate:

- Better SDL support for the upcoming years.
- Better window and video handling - we can go fullscreen and out without losing the OpenGL context!
- Keyboard input has Unicode support... although we might never really need to use this.
- It doesn't hurt!*

The libraries have changed, but the updated CMakeLists.txt should do the trick. Since we're using OpenGL, we didn't even have to create a Renderer, and a great deal of the existing engine code was left untouched.

Note: I seriously tried hard not to have the irrelevant commits in the pull request, but they kept sticking around. I wonder why... anyway, commit d903be1 is the only one relevant here.