Split date types into calendar and economy dates

See: 735abfe1

src/load_check.h

@@ -31,7 +31,7 @@ struct LoadCheckData {
 	std::string error_msg; ///< Data to pass to SetDParamStr when displaying #error.
 
 	uint32_t map_size_x, map_size_y;
-	Date current_date;
+	CalTime::Date current_date;
 
 	GameSettings settings;