Add game replay and undo/redo

[Mikolaj]

See AllureOfTheStars/Allure#50. One point is to have this feature, more or less powerful, another is to refactor the engine with this feature as a measure of success.

[Mikolaj]

For moves that revealed anything, allow undo and redo, just disallow restarting the game from any time point except point 0. So, if some combat animations scrolled past too quickly or were skipped by pressing a key, they can be replayed. Or a particularly masterful strike executed by the player.

[Mikolaj]

In principle, this should now be implementable without the need for any major code overhauls. However, the feature would not be usable at this point, because experiments show the stored atomic commands take too much memory and disk space. At first, we can just implement this feature and keep it disabled by default. Later on, something shrewd has to be done (probably the obvious optimizations won't solve the problem).

Edit: The best start is probably game replay (even though both use the same internal machinery), since undo/redo can lead to corrupted game state more easily, so may require more bugfixing.