make XmlAttributeComponent into a generic class

This change brings increased type safety to uses of
XmlAttributeComponent. Now the compiler is checkign for us that the
properties that get passed in to every subclass match the intended
interface, and also that the xmlKeys property -> xml attribute mapping
has all the right keys

ts/styles/style/components.ts

@@ -4,10 +4,8 @@ interface IComponentAttributes {
     val: string;
 }
 
-class ComponentAttributes extends XmlAttributeComponent {
-    constructor(properties: IComponentAttributes) {
-        super({val: "w:val"}, properties);
-    }
+class ComponentAttributes extends XmlAttributeComponent<IComponentAttributes> {
+    protected xmlKeys = {val: "w:val"};
 }
 
 export class Name extends XmlComponent {

ts/styles/style/index.ts

@@ -14,15 +14,13 @@ export interface IStyleAttributes {
     customStyle?: string;
 }
 
-class StyleAttributes extends XmlAttributeComponent {
-    constructor(properties: IStyleAttributes) {
-        super({
-            type: "w:type",
-            styleId: "w:styleId",
-            default: "w:default",
-            customStyle: "w:customStyle",
-        }, properties);
-    }
+class StyleAttributes extends XmlAttributeComponent<IStyleAttributes> {
+    protected xmlKeys = {
+        type: "w:type",
+        styleId: "w:styleId",
+        default: "w:default",
+        customStyle: "w:customStyle",
+    };
 }
 
 export class Style extends XmlComponent {