Introduced unit triggers!

Added triggered uniques to grant specific tech / policy

Civilopedia does not crash when displaying techs on fresh start

'upon declaring friendship' triggers for both sides

all unpillagable improvements are not destroyed by 'destroy improvements' unique

By SomeTroglodyte:
- Fix broken random nations pool
- City Screen displays "free" tile yields undimmed

AbsoluteUnits - Chu-Ko-Nu, Longbowman, Crossbowman v2  - By letstalkaboutdune