Z3c-formui

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- Feature: Added subform content and layout template. This allows you to
configure real sub forms which do not render the form tag.

- Feature: Improve layout implementation, support built-in layout templates.

- Feature: Use ``IContentTemplate`` instead of ``IPageTemplate`` in layout
base classes. This will help to prevent running into recursion errors if
there is a missing layout template.

- Feature: Added form module which offers built-in layout support.

- Bug: Added missing display ``IContentTemplate``, otherwise we can run into a
recursion in some cases.

- Bug: Renamed table macro argument from ``form-required-info`` to
``required-info``. The macro ``form-required-info`` did not exist.

- Bug: Added unit tests for layout support.

- Bug: Added tests for layout macros.

- Bug: Added tests for layout templates.

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- Refactoring: Update CSS classes to reflect latest changes to the widget
classes in ``z3c.form``.

- Bug: Error view snippets may have a empty ``widget`` attribute values, so we
cannot rely on accessing the label of the widget. This is the case, if the
error view sniipet was created from an invariants validation error.

- Bug: The table-form template did not properly render the error at the
widget, because the ``render()`` method was not called. Thanks to Markus
Leist for the report.

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- Feature: The row div element now also has an id of the form
"<widget-id>-row".

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- Refactoring: Split up registrations for simpler management of UI
components. This also makes it easier to see for developers how to create a
new template for forms.

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- Feature: Registered all defined macros for each form template. Also, added
more slots to the templates providing more hooks for customization.

- Feature: Added a macro/slot for the "required info", which explains how
required fields are marked.

- Feature: Added support for form labels.

- Feature: Added support for groups to templates.

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- Bug: Make sure we use the id for the "for" attribute of the "label"
element and not the name. This has worked until recently, because the
name and id were the same, but they are different now.