Moddable UI Skins [Basics] (#7714)

* Made UI skins selectable

This allows mods to provide alternative skin pngs without overriding the default skin purely by being downloaded

* Added UI Skin loading from NinePatches

This allows mod creators to define the stretch region and padding directly in the png

* Update baseScreen skin on skin reload

* Merged displayTab onChange functions

As all of them are equal anyway

core/src/com/unciv/Constants.kt

@@ -81,6 +81,7 @@ object Constants {
     const val dropboxMultiplayerServer = "Dropbox"
 
     const val defaultTileset = "HexaRealm"
+    const val defaultSkin = "Minimal"
 
     /**
      * Use this to determine whether a [MapUnit][com.unciv.logic.map.MapUnit]'s movement is exhausted