Alt+F4 to close the game on all branches

Grocel asked to enable Alt+F4 on all branches, since it only closed the game on x86-64. robotboy655 explained that Alt+F4 is blocked on all branches, but x86-64 sends quit because it doesn't check the key state when discarding WM_SYSCOMMAND. The request was closed the same day; commit efb205 enables ALT+F4 closing the game.