Session file fixer

[sgpearse]

We often have issues where a given solution is turned down because it will break legacy session files. This prevents us from making changes that would be ideal such as in #3116, where changing our Random Seed Bias from an int to a float introduces this scenario.

In the vein of providing better error messages, it might be worth considering mechanisms that do this heuristically or allow for manual XML/session.vs3 editing by the user.