Replacing SDL1.2 with SDL2.0 [RFC]

This branch contains minimal changes to replace SDL1.2 with SDL2.0. It should behave exactly like the older version except for internal changes in SDL2.

SDL touches many parts of the system and is important for a user therefor it should be tested thoroughly. Testing, comments and patches welcome.

Known Bugs:
* mouse input lags: I didn't touch the mouse movement code at all, will investigate further
* ...
There is no need to include <GL/gl.h>, SDL will include it for us
glu.h is located in OpenGL/glu.h afaik