Solved AI Great Scientist crash

Great Merchant doesn't try to go to unreachable tiles

By letstalkaboutdune:
- AbsoluteUnits - Sipahi, Hakkapeliitta, Janissary, Tercio, Musketeer, Minuteman

By SomeTroglodyte:
- Make AI diplomatic marriage safe from concurrent modification issues
- Fix Main Menu BG map cycle not stopping on user action