Modding: terrainFilter now accommodates nationFilter for tile owner

Minor UI improvements

Performance improvements  - By WhoIsJohannes

Allow to reveal explored resources from a city's demanding resources in `CityOverviewTab`  - By chr56

By SomeTroglodyte:
- Visual clue a load game from clipboard is underway
- Allow modders to use culture/faith conversion without providing the icons
- "can be promoted" notification only when it's actually new
- Fix wrapping for promotions in unit overview