3 classes represent the boolean logic for a `<level>` tag, this CSettingLevel is a container for those levels.

IOW, `<visibility>true</visibility>` gets 3 classes, but
```xml
<visibilities attributes="2">
    <visibility value="0">true</visibility>
    <visibility value="1">false</visibility>
</visibilities>
```
needs a fourth class to manage its attributes and the list of visibilities that it contains.