Include widget info in JSON Schema produced by @types endpoint

[lukasgraf]

The JSON Schema produced by @types/[portal_type] currently only includes the field type, but no information about which widget should be used. This information should be included in the JSON Schema as well.

A custom widget for a field can be specified using a plone.autoform directive. That information then gets stored on the schema interface using tagged values, which should be easy to access and serialize.

The default widget however for a particular zope.schema field type is currently being determined by z3c.form, which means we might need to replicate some of z3c.form's logic.

/cc @ebrehault

[tisto]

We need this as well soonish. We already include a widget attribute for some fields but not for all. Will look into it.

[ebrehault]

In angular2-schema-form, we use a widget property which is either a string (the id of the widget), either a dictionary if we need widget settings (like for a date/time, tinymce, etc.). See
https://github.com/makinacorpus/angular2-schema-form#widgets

[tisto]

@ebrehault using the widget property is fine. I just would make sure the property is always included and not "None" as it is now.

[tisto]

Pull request: #205