Game action recording / replay system

We'll need to be able to save all the player actions taken during the game, and replay them using the same data and time stamps they were using while playing.

This is useful to watch other players' recordings, or validating the game records.

This won't be an easy task, I believe, but we still need it.

Each recording should be saved with a game version (and maybe format version) in its header, for us to be able to emulate older formats in the future (if required).