Add Undo Button

[averyzheng-yay]

May be hard to do with current setup.

[npvq]

An Undo Button might not be necessary for our version of minesweeper.

When the player clicks a given tile, it could either be a mine or reveal a number (possibly zero/empty).

• If the tile is a mine, the player should lose the game immediately.
• If the tile reveals a number, the player has no reason to undo/revoke the gained information.