PLIP 13476 mockup - use plone.app.widgets instead of c.z3cform.datetimewidget

For Date and Datetime fields, use plone.app.widgets by default.
Besides that, use plone.app.testing instead of deprecated test layers and PEP8 everything.

This pull-request can be be merged in, when plone.app.widgets gets released and merged into Plone5.