Inspection - unused variable

Closes #21

First pull request for this change was in #90. Second attempt, with some improvement done in #97.

This is same as last version of #97, but it should not indicate that many changed files.

* better names of sample functions in tests
* smaller better named functions in UnusedVariableFinder
* added some additional support for variable defined in for clause of FLWOR expression
* basic handling of variable namespaces (by using prefix)

Not supporting global private variable yet - possible in another commit/another issue.