WIP #974 "XPath dependencies: support xxf:r()"

- don't recompute xml:lang references (lazy val holding LangRef in ElementAnalysis.lang)
- xml:lang search uses ElementAnalysis, not Element
- createElementAnalysis of xf:choice, xf:itemset, xf:item, xf:label and xf:value
- keep static itemsets after first evaluation
- move XXFormsFormatMessage and XXFormsLang to Scala
- enable unit test of call to xxf:lang() in variable value
- analysis toXML is hierarchical
Fix imports
XPath dependencies: xxf:lang() and xxf:format-message()

Also:

- tests
- remove argument to xxv:lang(), which is not used in Orbeon Forms and not documented
FB: Remove unneeded ref to model not in scope