Improved clarity and moddability of building improvements (#6712)

* Renamed tile.hasUnique, deprecated `Indestructable`, unique for citadels
Also refactored the consumption of (great) people out of UnitActions.

* Reworked when improvements can be build somewhere for more clarity

* Made resources improvable by multiple improvements; Offshore Platform

* Fix compatability

* WIP

* Fixed the tests, but better

* I suppose I might as well update this now that we're a version later

core/src/com/unciv/Constants.kt

@@ -30,6 +30,7 @@ object Constants {
     val vegetation = arrayOf(forest, jungle)
 
     // Note the difference in case. **Not** interchangeable!
+    // TODO this is very opaque behaviour to modders
     /** The "Fresh water" terrain _unique_ */
     const val freshWater = "Fresh water"
     /** The "Fresh Water" terrain _filter_ */