Allow highest level menu definitions - typically used for root-menu and
client-menu - to be defined like this:
<openbox_menu>
<menu id="">
</menu>
</openbox>
Previously this required a label attribute (which was not used for
anything and could be an empty string) as show below:
<openbox_menu>
<menu id="" label="">
</menu>
</openbox>
Closes issue #472