Fix lock screen shortcut key bind in Lubuntu 13.10

After upgrade to Lubuntu 13.10. The Ctrl-Alt-L key combo defined in ~/.config/openbox/lubuntu-rc.xml bind to xsreensaver-command -lock is no longer working because xsreensaver is not installed. Instead, we should replace it with lxlock. Edit ~/.config/openbox/lubuntu-rc.xml:

    <!-- Lock the screen on Ctrl + Alt + l-->
    <keybind key="C-A-l">
      <action name="Execute">
        <command>lxlock</command>
      </action>
    </keybind>