Ruleset validator: Tilesets (#9881)

* Defensive Tileset loading

* Deleting a mod should do the same cleanup as downloading a new one

* Extensive RulesetValidator checks on Tileset mod integrity

core/src/com/unciv/Constants.kt

@@ -90,6 +90,8 @@ object Constants {
     const val uncivXyzServer = "https://uncivserver.xyz"
 
     const val defaultTileset = "HexaRealm"
+    /** Default for TileSetConfig.fallbackTileSet - Don't change unless you've also moved the crosshatch, borders, and arrows as well */
+    const val defaultFallbackTileset = "FantasyHex"
     const val defaultUnitset = "AbsoluteUnits"
     const val defaultSkin = "Minimal"