quickSettings: Add getFirstItem() method

We will need some more access to the menu's underlying grid to
provide extension API for adding additional quick items.

Expose a new getFirstItem() method that (surprise!) returns the
first item.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2894>

js/ui/quickSettings.js

@@ -802,6 +802,10 @@ export const QuickSettingsMenu = class extends PopupMenu.PopupMenu {
         }
     }
 
+    getFirstItem() {
+        return this._grid.get_first_child();
+    }
+
     open(animate) {
         this.actor.show();
         super.open(animate);