Z3c.form

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- *MAJOR* overhaul of ObjectWidget:

* low level unittests passed, but high level was not tops
basic rule is that widgets want RAW values and all conversion
must be done in ``ObjectConverter``

* ``ObjectSubForm`` and ``SubformAdapter`` is removed,
it was causing more problems than good

* added high level integration tests

- Removed ``z3c.coverage`` from ``test`` extra. [gforcada, maurits]

-------------------

- RadioWidget items are better determined when they are needed [agroszer]

- CheckBoxWidget items are better determined when they are needed [agroszer]

- Bugfix: The ``ChoiceTerms`` adapter blindly assumed that the passed in field
is unbound, which is not necessarily the case in interesting ObjectWidget
scenarios. Not it checks for a non-None field context first. [srichter]

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- Correctly handled ``noValueToken`` in RadioWidget. This avoids a
``LookupError: --NOVALUE--``. [gaudenz,ale-rt]

- Added ``json`` method for forms and ``json_data`` method for
widgets. [mmilkin]

- Change javascript for updating ordered select widget hidden structure so it
works again on IE11 and doesn't send back an empty list that deletes all
selections on save. Fixes https://github.com/zopefoundation/z3c.form/issues/23
[fredvd]

- Started on Dutch translations.
[maurits]

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- Standardized namespace ``__init__``. [agroszer]

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- Remove "cannot move farther up/down" messages
in ordered select widget.
[esteele]

- Updated Traditional Chinese translation.
[l34marr]

------------------

- Fixed warnings in headers of locales files.
Checked with ``msgfmt -c``.
[maurits]

- Added Finnish translation.
[petri]

- Added Traditional Chinese translation.
[l34marr]