Settings API: allow conditional levels

Settings level can either be a simple level

<level>2</level>

or a more complex one

<levels default="2">
  <level value="0">
    <and>
      <condition>true</condition>
      <condition>false</condition>
    </and>
  </level>
  <level value="1">
    <and>
      <or>
        <condition>false</condition>
        <condition>true</condition>
      </or>
      <condition>true</condition>
    </and>
  </level>
</levels>
Modify settings.xml suitable for <level> tests