Spiritus Astrum [P0-B13]: p1536: Towards Early Access

I have spent a lot of time adding in a checkpoint save system for the third campaign.

This allows the player to quit and resume the game at the most recent checkpoint.

The system is quite basic, and needs a lot of testing, but the core methodology is working.

There is one intermittent bug that occurs when the player returns to their capital ship, but I have not yet been able to track this down. This only seems to occur during checkpoint loading.

I also fixed a few minor bugs that I found when working on the checkpoints.

There is still quite a bit to do. The next goal is to add checkpoints to the other campaigns. This should be easier, since they are more procedural and less objective based.