Re-add 'construct great improvement' automation for great units that can't do their main actions

Promotion added to unit already containing that promotion, does not retrigger trigger uniques

"Unavailable" units cannot be upgraded to

By SeventhM:
- Consider passive strength bonuses for force value 
- Pass in civ for building on tiles 

By SomeTroglodyte:
- Larger clipboard size on Desktop
- Add a Unique allowing an Improvement to specify which Resource(s) it improves