EMI: Save sortplanes (RFC)

Currently loading savegames sometimes break, as the sortplanes aren't saved or loaded, there are two possibilities for fixing this:

1: Do an ad-hoc savegame-version update (good), which is what this PR does
2: Disable sortplane-usage after loading, untill we have enough savefixes to warrant an update.

In addition, this opens the question, of how we should version the savegames for our two games:
Should EMI-changes be allowed to change the version-number for Grim too, or should we introduce a double versioning scheme (probably harder to maintain).