add separate sort order field to levels, so they can be reordered without changing the id and invalidating solutions

src/main.rs

@@ -236,7 +236,7 @@ fn get_levels() -> Vec<Level> {
 			levels.push(level);
 		}
 	}
-	levels.sort_by(|a, b| a.id().cmp(b.id()));
+	levels.sort_by(|a, b| a.sort_order().cmp(&b.sort_order()));
 	levels
 }