Modding: Unbuildable units can still be upgraded to

Modding: Fallback image for modded buildings and techs

By vegeta1k95:
- Rework of City Screen
- Huge update of Technology Picker screen UI to match Civ 5

By FiretronP75:
- Continent and Islands  map
- Map editor responds to max zoom setting
- Map generation improvements (conflicting terrains, water generation)

AbsoluteUnits - Mongolian Uniques - By letstalkaboutdune

Added missing translation terms - By Ouaz