The previous regular expression based approach sometimes could not extract
message properly. Using xml parser simplify code and fix several messages
that were not extracted properly, like messages containing ", [] or {}

This also fix some problems when looking for messages sources:
  - archived web pages were sometimes used instead of published ones
  - messages from gadgets implemented as page templates/OFS files were not
    extracted.

A few more unit tests for the scripts involved in this process are added.