Added unitset selection to options menu (#7980)

* Added unitset selection to options menu

* Added None type to unitset selection

+ added logic to switch unitset when changing tileset as long as one with the same name exists and both are selected

* Updated default unitset to be HexaRealm

core/src/com/unciv/Constants.kt

@@ -81,6 +81,7 @@ object Constants {
     const val dropboxMultiplayerServer = "Dropbox"
 
     const val defaultTileset = "HexaRealm"
+    const val defaultUnitset = "HexaRealm"
     const val defaultSkin = "Minimal"
 
     /**