NetworkMenu: destroy More... submenu when empty

Add a .length property to PopupMenuBase, and use it from the network
menu to destroy the menu when removing a network without rebuilding.

https://bugzilla.gnome.org/show_bug.cgi?id=645981

js/ui/status/network.js

@@ -1198,6 +1198,11 @@ NMDeviceWireless.prototype = {
             if (apObj.item)
                 apObj.item.destroy();
             this._networks.splice(pos, 1);
+            if (this._overflowItem &&
+                this._overflowItem.menu.length == 0) {
+                this._overflowItem.destroy();
+                this._overflowItem = null;
+            }
         } else if (apObj.item)
             apObj.item.updateAccessPoints(apObj.accessPoints);
     },