feat(loading): add internal stack for the loading service

this allows the user to push multiple loading indicators on top of each
other without interfering with the page stack, as well as navigation
between pages behind the loading indicator.

references #5426

ionic/components/nav/view-controller.ts

@@ -77,6 +77,11 @@ export class ViewController {
    */
   isOverlay: boolean = false;
 
+  /**
+   * @private
+   */
+  usePortal: boolean = false;
+
   /**
    * @private
    */