Support withSubs on the menuproviders in appmenu

Adds support to define child providers for a parent menu using a .withSubs option. This parent menu will always be displayed as an expandable menu.
When no children are visible, the expandable menus will be hidden.

src/app/shared/menu/menu-section.model.ts

@@ -79,4 +79,6 @@ export interface MenuSection {
    * Note that not all menus may render icons.
    */
   icon?: string;
+
+  isExpandable?: boolean;
 }