Add a setting for randomly rotating boards on file load

[JiayingWei]

This is a feature that I've been working on

Use case: I like to create go sgf problems from my games, and I use a flashcard app to regularly review them. Upon opening I would like the board randomly rotated to make sure I'm not just memorizing the stone position.

Implementation:

• A new setting in Menu > Sabaki > Preferences called "Randomized board rotation on load"
• If checked, on file load the board will be rotated 0, 90, 180, 270 degrees

[maxzsol]

This is a great idea. Beyond rotating, I would suggest mirroring the game by flipping the board.
With mirroring and rotation one problem could be shown in 8 different variations.