CustomJXPathBindingBuilder provides a helper class for the Factory
implemented in JXPathBindingManager that helps construct the
actual CustomJXPathBinding out of the configuration in the
provided configElement which looks like one of the following:
<fb:custom id="widget-id" path="xpath expression"
builderclass="your.package.CustomBindingXBuilder"
factorymethod="makeBinding"
>
<fb:config custom-atts="someValue">
<!-- in here come the nested custom elements (recommended in own namespace)
that make up the custom config -->
</fb:config>
</fb:context>