Custom map size (#2876)

* Adding custom map sizes. Initial commit

* Custom map sizes UI update

* Custom map size with rectangular shape

* Added compatibility with older Maps and Game saves

* Fixed build errors and added warning messages

Co-authored-by: Yair Morgenstern <yairm210@hotmail.com>

core/src/com/unciv/Constants.kt

@@ -76,4 +76,11 @@ object Constants {
     const val futureEra = "Future era"
     const val barbarians = "Barbarians"
     const val spectator = "Spectator"
+
+    const val tiny = "Tiny"
+    const val small = "Small"
+    const val medium = "Medium"
+    const val large = "Large"
+    const val huge = "Huge"
+    const val custom = "Custom"
 }