- in _create_section_from_dict, the context is the nearest "section"
  which can be a survey, group, or repeat. When it's not the survey,
  the survey-level choices dict isn't directly available. So this is
  now tracked by the builder as a class-level object.
- added tests for groups, repeats, groups-in-groups, repeats-in-groups.

- standardise to help avoid misspelling / refactoring issues in future

- warning is once per form. Re-used missing translations check code
  to avoid doing the same processing twice. The new check needed the
  intermediate result of the missing check.
- test_validators.py: removed version check for import since supported
  versions have been above 3.3 for a while.