Modding: Humidity/Temperature limits on terrain features

Allow mass unit upgrades from the unit overview screen

Calculate movement correctly for road + railroad combinations

Modding: Workers pick the best improvement for resources

By FiretronP75:
- Add random select for map options
- Fixed flat world generation

By vegeta1k95:
- Resource icons colored by type
- Modding: allow custom TechPortraits
- Fixed PolicyScreen branches requirements text

Translate "Sell" in city screen  - By Ouaz