NetworkMenu: take out an item from More... when another is destroyed

When one of the networks in the main menu is removed and we have
a More... submenu, we can take the first out from the submenu and
show it in the main menu.

https://bugzilla.gnome.org/show_bug.cgi?id=647175

js/ui/popupMenu.js

@@ -946,6 +946,14 @@ PopupMenuBase.prototype = {
         });
     },
 
+    get firstMenuItem() {
+        let items = this._getMenuItems();
+        if (items.length)
+            return items[0];
+        else
+            return null;
+    },
+
     removeAll: function() {
         let children = this._getMenuItems();
         for (let i = 0; i < children.length; i++) {