[widgetexplorer] Don't recurse into applet's containments

The intention of this code appears to be finding system tray applets, since the systemtray is a containment within another applet/containment

However the code causes an infinit recursion since we are calling addContainment on the same containment again

The recursion also is unnecessary since corona->containments() already lists the system tray containment, so we don't need to find it by recursing through the panel's children