Fix nxu:valueHolder tag exposure of value on ajax rerender

The nxu:valueHolder tag does not seem to expose its value correctly unless it's doubled by a nxu:set tag with attribute cache="true" in the case of the re-rendering of selected flavor in theme local configuration for instance (see /widgets/select_flavor_widget_template.xhtml).

This workaround cannot work inside a nxu:inputList widget because the cache="true" forces the value to be exposed at build time, and the list widget exposes each of the elements value at render time.