Add: Setting to save a unique id for each savegame

src/saveload/afterload.cpp

@@ -3045,6 +3045,12 @@ bool AfterLoadGame()
 		}
 	}
 
+	if (IsSavegameVersionBefore(203)) {
+		/* Generate a random id for savegames that didn't have one */
+		/* We keep id 0 for old savegames that don't have an id */
+		_settings_game.game_creation.generation_unique_id = _interactive_random.Next(UINT32_MAX-1) + 1; /* Generates between [1;UINT32_MAX] */
+	}
+
 	/* Station acceptance is some kind of cache */
 	if (IsSavegameVersionBefore(127)) {
 		Station *st;

src/saveload/saveload.cpp

@@ -270,8 +270,9 @@
  *  200   #6805   Extend railtypes to 64, adding uint16 to map array.
  *  201   #6885   Extend NewGRF persistant storages.
  *  202   #6867   Increase industry cargo slots to 16 in, 16 out
+ *  203   #6973   Add unique id to savegames
  */
-extern const uint16 SAVEGAME_VERSION = 202; ///< Current savegame version of OpenTTD.
+extern const uint16 SAVEGAME_VERSION = 203; ///< Current savegame version of OpenTTD.
 
 SavegameType _savegame_type; ///< type of savegame we are loading
 FileToSaveLoad _file_to_saveload; ///< File to save or load in the openttd loop.