Moved SDLSys and win32 into src/OS

The various OS specific files are now in their own folders.

Also SDLSys now implements a stub method for KeyDown to make it a
concrete type.
main.cpp reflects the move of the OS files
Made CMake more per-platform

The CMake files now change the including source and libraries based on
the OS choice.
Made GLSPrite aware of SDL and SOIL

GLSprite now uses the appropriate library to load images.