Add cancel button to New Game State menu

This is my first attempt at contributing to an open source project so shoot me if it's trivial :), but I figured this cancel button is nice because I used to hate (still do with OpenXcom) when I mis-clicked New Game when I meant to do Load Game because I had to do the following:

Click a difficulty
Click a place on the map
Enter a bogus name
Hit Enter
Click Options
Click Abandon Game or click Load Game
Click new game to load or OK if Abandon game...

just to get back to the main menu or load the game I meant to.