Reduce RAM when saving/loading to/from files

Fix rare crashes when handling events while screens are being changed

Fix crash on Options re-open with force-close, e.g. toggling "Enable out-of-game turn notifications" - By SomeTroglodyte 

AI does not see unexplored tiles for tile value when assessing settler locations - By ssamt 

Clarify distance-based unit cycle option - By mvanhorn

Population in cities countable - By PLynx01

Modding: Downgrade "same name for multiple entries" to "OK", lowest level of warning, since most cases are benign