Plone.app.z3cform

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

New features:

- Link widget: add ``placeholder`` attributes for external and email link input fields.
[thet]

Bug fixes:

- Fix: Add missing i18n-domains to templates.
[jensens]

- Use RichTextValue's output_relative_to(self.context) in RichTextWidget so the ITransform doesn't use siteroot.
[jaroel]

- Fix in link widget data converter for ``toWidgetValue`` to return an empty structure when the field value is empty instead of returning the portal root object.
Fixes: https://github.com/plone/Products.CMFPlone/issues/2163
[thet]

- Keep "internal" links with query strings as external links, otherwise
the query string is lost
[tomgross]

- Allow an additional CSS class for widgets in this package
[tomgross]

- Document customization of widgets
[tomgross]

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

Bug fixes:

- Test fixes for changes in plone.app.widgets querystring options.
[thet]

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

New features:

- Add new and enhanced link widget.
[tomgross, thet]

Bug fixes:

- Fix broken ``get_tinymce_options`` when called with non-contentish contexts like form or field contexts.
[thet]

- Related Items Widget: In search mode, when no basePath was set, search site-wide.
Fixes: https://github.com/plone/mockup/issues/769
[thet]

- Fixes 64: SingleCheckBoxFieldWidget does not render value in view mode.
In order to fix this issue the hacky view was removed.
It is replaced by a new widget to render a single checkbox with bool values.
An appropriate data converter was added as well.
[jensens]

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

Breaking changes:

- Removed ``plone.app.z3cform.object`` and
``plone.app.z3cform.objectsubform`` because z3c.form 3.3 removed the
underlying code.
See https://github.com/zopefoundation/z3c.form/pull/38 for upstream changes.
[maurits]

New features:

- Add new class ``view-name-VIEWNAME`` to form element indicating the view name w/o old kss prefix.
New class's replaces ``++`` in view by ``-`` in order to produce valid class (CSS selectable) names.
[jensens]

Bug fixes:

- Catch TypeError occurring on conflicting subrequests in inline validation
[tomgross]

- Clean up: code-style, zca-decorators, replace lambda.
[jensens]

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

New features:

- Do not show the "Clear" button for required Date or DateTime fields.
[thet]

Bug fixes:

- Test fixes for plone.app.widgets 2.1.
[thet]

- remove deprecated __of__ for browserviews
[pbauer]

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

Breaking changes:

- Test fixes for plone.app.widgets 2.1.
While this is not a breaking change functionality or API wise, the tests do only pass with plone.app.widgets 2.1.
[thet]

Bug fixes:

- Fix RelatedItemsDataConverter with relation lists, where in an iteration a wrong value was checked to be existent.
Fixes failures in situations, where a ``None`` value was part of the relation list.
[thet]

- Fix RelatedItemsDataConverter with choice lists, where choices are UUID
strings of selected relations, but conversion failed, because Choice
field has None as its value_type
[datakurre]