Optimize retrieval of values used by JSF iterations

When using a getter to get a list of documents, or a select model, or result providers, the same call is done a lot of times because of JSF iteration that resolves the property again and again.

Attribute immediate="true" that was set on tag nxu:methodResult makes it possible to resolve it only once, so it would be useful to get a more generic tag about all kinds of variables.

nxu:methodResult would benefit from it too since it is not useful anymore with jboss-el, resolving methods on its own.