ValueJXPathBindingBuilder provides a helper class for the Factory
implemented in JXPathBindingManager that helps construct the
actual ValueJXPathBinding out of the configuration in the
provided configElement which looks like:
<wb:value id="widget-id" path="xpath-expression">
<!-- optional child binding to be executed upon 'save' of changed value -->
<wb:on-update>
<!-- any childbinding -->
</wb:on-update>
</wb:value>