Extensive performance work

Can no longer gift units to enemy City States

Can now have owner-style-specific improvement images (e.g. Mine-Medieval-European)

Resolved rare crashes

By Azzurite:
- Create turn notifier for when the game is running for Windows
- Fix Sweden not being able to gift great people to city states

By Jack Rainy:
- Allow trade routes via city-states
- Better unit expulsion when declaring war/after open borders ended

New Demographics Scoreboard  - By letstalkaboutdune