Introduce some functional programming techniques

src/file/drawing/effect-extent/effect-extent-attributes.ts

@@ -1,14 +1,14 @@
 import { XmlAttributeComponent } from "file/xml-components";
 
 export interface IEffectExtentAttributes {
-    b?: number;
-    l?: number;
-    r?: number;
-    t?: number;
+    readonly b?: number;
+    readonly l?: number;
+    readonly r?: number;
+    readonly t?: number;
 }
 
 export class EffectExtentAttributes extends XmlAttributeComponent<IEffectExtentAttributes> {
-    protected xmlKeys = {
+    protected readonly xmlKeys = {
         b: "b",
         l: "l",
         r: "r",