Not so unique improvements 1.03

Fixed: only 1 type of unique improvement per city was placed when city was conquered.
Added: unique improvements not removed when city is gifted or changes owner because of low loyalty.
Added: city states' unique improvements.
Added: City states can build unique improvements they provide.

Added workaround for game bug that causes some DLC improvements not being displayed.

Fixed bug that caused unique improvements appear instead of regular ones.