Autofocus onto the game window

Normally, you need to click the game window before you can control the
player with the keyboard. This commit adds functionality such that the
game window is autofocused upon loading so you can begin controlling the
player immediately. It also autofocuses once you click the Replay button.
Autofocus on the game window on-load

Normally, you'd need to click on the game screen before controlling the
character. This commit added functionality such that the game window will
be "autofocused", meaning you can begin controlling the player immediately
after loading. It also re-focuses on the game screen when the Replay
button is clicked to restart the stage.