Add support for all plone.autoform tagged values

[tisto]

This is a follow up of #198. We should support all tagged values that are defined in plone.autoform:

MODES_KEY = u"plone.autoform.modes"
OMITTED_KEY = u"plone.autoform.omitted"
ORDER_KEY = u"plone.autoform.order"
WIDGETS_KEY = u"plone.autoform.widgets"

READ_PERMISSIONS_KEY = u"plone.autoform.security.read-permissions"
WRITE_PERMISSIONS_KEY = u"plone.autoform.security.write-permissions"

https://github.com/plone/plone.autoform/blob/master/plone/autoform/interfaces.py#L13

[lukasgraf]

I took this up at the Beethoven Sprint, and while for certain plone.autoform directives (omitted, mode, permissions) it's as simple as just exposing the tagged values (assuming IRO has been properly honoured), that's not the case for fieldset and field_order directives.

Those require some advanced interpretation in order to achieve a meaningful result. I therefore overhauled the way that the JSON schemas are built in the @types endpoint, see #237

[tisto]

Merged in #237