'Crop yield' chart displays correct data

Modding:
- Units can transform to any unit
- Fixed policy softlock from conflicting mod policies
- 'Unpurchaseable' units are properly unpurchaseable

By SomeTroglodyte:
- Map editor update - concurrency, resource amounts, file double-click
- Fix road maintenance unique being ignored
- Fix mouse wheel on Notifications scroll
- Unit overview remembers scroll position after promoting

Allow game loading even when no saved games exist - By alexban011