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/numbering/level.ts

@@ -7,14 +7,11 @@ interface ILevelAttributesProperties {
     tentative?: number;
 }
 
-class LevelAttributes extends XmlAttributeComponent {
-
-    constructor(properties: ILevelAttributesProperties) {
-        super({
-            ilvl: "w:ilvl",
-            tentative: "w15:tentative",
-        }, properties);
-    }
+class LevelAttributes extends XmlAttributeComponent<ILevelAttributesProperties> {
+    protected xmlKeys = {
+        ilvl: "w:ilvl",
+        tentative: "w15:tentative",
+    };
 }
 
 class Start extends XmlComponent {