add richtext field/widget support and remove Products.TinyMCE dependency

This fix https://github.com/plone/mockup/issues/231.
This pull requests comes with two others that need to be merged together:
 1. https://github.com/plone/plone.app.contenttypes/pull/116/commits
 1. https://github.com/plone/plone.app.dexterity/pull/111/commits