Make it possible to resolve expressions twice using nxu:set

Sometimes we'd like an expression to be resolved twice. The obvious use case is actions management, where the xhtml templates will display #

. Since action configuration might want to reference another expression, it is useful to resolve expressions twice.

This already handled natively by some specific Nuxeo tags like nxh:commandLink or nxh:outputText, but it would be better to make it independant from the tag => the tag handler nxu:set, that handles variables resolution and aliases, is a better candidate.