Cmake + MinGW

Hopefully improved compatibility with MinGW
Spelling fix and change in SDLSys::MessageLoop()

KeyboardEventSystem::KeyDown should now only fire once when the key is
pressed, otherwise my camera accelerates the rotation every tick.
Also KeybaordEventSystem was renamed to KeyboardEventSystem
Undo the change to DSLSys::MessageLoop()
Typo and MinGW fixes

Added some CMake checks for MinGW to use SDL over Win32/

Fixed a typo in KeyboardEventSystem.
Fixed SDL::GetDeltaTime()

GetDeltaTime() now properly stores the last time to use when compute the
current delta.