Implement replay seeking

[nmlgc]

Why should we just have fast-forwarding? Let's implement full seeking across an entire replay, complete with rewinding and single-frame stepping features.

The most robust way of implementing rewind will probably involve snapshots of the entire game state at regular keyframe intervals, which we can then use as a base for re-simulating any frames inbetween. This requires a great deal of refactoring to move all of the currently global game state into a structure.

The main reasons for doing this in-engine:

• Debugging, allowing devs to seek back to a previous frame and step through its code.
• We could have another feature where the player can take control at any frame of the replay, implementing some sort of basic thprac functionality.

If you just want to watch a replay though, video export would be a cheaper alternative.