Add z3c.form implementation of folder_constraintypes_form

Tests are in plone.app.dexterity for now, because they're p.a.testing-based
and incompatible with the PloneTestCase tests in here currently.
on the folder_contents view, show the add menu for the
  context object always. This fixes the issue when the
  default view of a folder is also a folder and you
  can not add items to it. fixes https://dev.plone.org/ticket/9057
show a warning message on the folder contents view when
  the default page is also a folder, that in order to add items
  to the default page's folder, they'll need to visit it's
  folder_contents view. also addresses https://dev.plone.org/ticket/9057
also on folder factories, show correct add items
Preparing release 2.1
Back to development: 2.1.1
Add a dependency for tests using plone.app.testing
In the folder_contents view always set the base tag with a trailing slash.

Fixes https://dev.plone.org/ticket/13487
Merge pull request #4 from danjacka/danjacka-basetagfix

In the folder_contents view always set the base tag with a trailing slash.
merge origin/master