add option to generate game with random advantage

[wanqizhu]

I find it useful to study board evaluation by looking at a random middle game board and guessing who's ahead and by how much. You can do this now with setup game by choosing a random advantage (and hiding the scoreboard UI so it doesn't tell you the answer). There's a keyboard shortcut ; to do this repeatedly.

I couldn't get the UI to look very nicely though, would appreciate some help on that.