3n opening method for handicap games

[kaorahi]

I implemented 3n opening method for handicap games as a trial.

https://github.com/kaorahi/lizgoban/tree/3n_handicap_220321a

I have no plan to release it because it adds some complexity to the code. Please let me know if you want to change my mind.

[kaorahi]

You can try this in lizgoban v0.7.0-pre3. (Just this version. It will be removed in the next release.)

https://github.com/kaorahi/lizgoban/releases/tag/v0.7.0-pre3

Usage:

1. Click "File > Match vs. AI".
2. Select "Tool > Experimental > 3n handicap > n = 4", for example. You will see randomly selected 3n (= 12) grids (blue square marks) on the board.
3. Click any n squares on the board to start the game.