[1.x] [extensibility] refactor(core): improve extensibility of IndexPage (#4045)

* refactor(core): improve extensibility of `IndexPage`

* refactor(core): refactor the extensibility refactor

Author: DavideIadeluca

framework/core/js/src/forum/components/IndexPage.tsx

@@ -60,19 +60,31 @@ export default class IndexPage<CustomAttrs extends IIndexPageAttrs = IIndexPageA
             <nav className="IndexPage-nav sideNav">
               <ul>{listItems(this.sidebarItems().toArray())}</ul>
             </nav>
-            <div className="IndexPage-results sideNavOffset">
-              <div className="IndexPage-toolbar">
-                <ul className="IndexPage-toolbar-view">{listItems(this.viewItems().toArray())}</ul>
-                <ul className="IndexPage-toolbar-action">{listItems(this.actionItems().toArray())}</ul>
-              </div>
-              <DiscussionList state={app.discussions} />
-            </div>
+            <div className="IndexPage-results sideNavOffset">{this.contentItems().toArray()}</div>
           </div>
         </div>
       </div>
     );
   }
 
+  contentItems(): ItemList<Mithril.Children> {
+    const items = new ItemList<Mithril.Children>();
+
+    items.add('toolbar', <div className="IndexPage-toolbar">{this.toolbarItems().toArray()}</div>, 100);
+    items.add('discussionList', <DiscussionList state={app.discussions} />, 90);
+
+    return items;
+  }
+
+  toolbarItems(): ItemList<Mithril.Children> {
+    const items = new ItemList<Mithril.Children>();
+
+    items.add('view', <ul className="IndexPage-toolbar-view">{listItems(this.viewItems().toArray())}</ul>, 100);
+    items.add('action', <ul className="IndexPage-toolbar-action">{listItems(this.actionItems().toArray())}</ul>, 90);
+
+    return items;
+  }
+
   setTitle() {
     app.setTitle(extractText(app.translator.trans('core.forum.index.meta_title_text')));
     app.setTitleCount(0);