fix(command/ExitGame): ExitGame by sending quit message through SDL2

[EnderHane]

fix #128

[EnderHane]

This method let the game exit gracefully, FNA being able to handle resources cleaning and complete process lifecycle.

Alternatives:

1. System.Environment.Exit() or System.Environment.FailFast()
   not good :<
2. send SDL_WINDOWEVENT_CLOSE
   requires WindowID, thus being more too complicated

[psyGamer]

What is the usecase for ExitGame today?
I honestly thought I had already removed the command entirely.