Plone.app.z3cform

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

Bug fixes:

- Fix deprecation warning
(https://github.com/plone/Products.CMFPlone/issues/2605) [ale-rt]

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

Bug fixes:

- Prepare for Python 2 / 3 compatibility
[pbauer, MatthewWilkes, ale-rt]

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

Bug fixes:

- Cleanup code analysis problems.
[jensens]

- Fix SingleCheckBoxBoolWidget description to render structure
[allusa]

- Prepare for Python 2 / 3 compatibility
[pbauer, MatthewWilkes, ale-rt]

- Render mimetype selection box correctly for a ``RichTextWidget`` which also
brings back the TinyMCE.
[sallner]

- Allow RelatedItems widget to be used in subforms
[tomgross]

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

Bug fixes:

- Fix test failures from https://github.com/plone/plone.app.widgets/pull/177
[thet]

- Prepare for Python 2 / 3 compatibility
[pbauer]

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

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]