Settings API: allow conditional levels

Settings level can either be a simple level

<level>2</level>

or a more complex one (in this case, evaluating to "1"):

<levels default="2">
    <level value="0">false</level>
    <level value="1">
        <and>
            <condition>true</condition>
            <or>
                <condition>false</condition>
                <condition>true</condition>
            </or>
        </and>
    </level>
</levels>