Investigator footprints list manipulation is very complicated

[Anusien]

The Investigator has this Footprint class. It manages its own lifecycle by hooking into the Investigator class. It has a lot of logic to add itself to the list of footprints when created and remove itself when it's aged out. This ends up with some complicated logic to iterate through the list of footprints where the for loop has to manage its own index variable and move it backward because some functions will remove themselves.

We should clean all this up so the code that adds and removes things from the list also controls insertion. I suspect this moves all the logic to AddPrints and makes Footprint way more simple.