Add schemaeditor support

This pull adds TTW configuration for relation choice and list field with "allowed target types" setting.

This pull doesn't have any tests yet and is missing p.a.widgets support. Also, this pull introduces dependency on plone.app.vocabularies and conditionally restores configuration for plone.formwidget.contenttree.

It would be nice, if someone familiar with supermodel and schemaeditor could review, is this the proper way to do this, before I finish this (with p.a.widgets support).

I want to be able to define relation choice and relation list fields TTW using the schema editor. Also, I need this still to work on Plone 4 without p.a.widgets and therefore I must support plone.formwidget.contenttree by default (when plone.formwidget.contenttree is present but plone.app.widgets is not).