Integrated sdl2-cs bindings for sdl2

This is using the sdl2/pure branch of https://github.com/thefiddler/SDL2-CS.git which contains the SDL2 bindings without MiniTK.
Implemented SDL2 platform using sdl2-cs

This is a new platform that can be used then sdl2 is installed on the target system. SDL2 is commercially supported by Valve and provides better support for MacOS compared to our current implementation (Cocoa vs Carbon). It will also help us introduce faster support for new platforms. Existing platforms remain as a fallback and will be automatically used if sdl2 is not installed.

Please note that this is still a work in progress. The new mouse and keyboard API is not supported yet. Due to limitations of sdl2, multiple mice/keyboards are also not supported.