import cleanup
- Make use of new z3c.form DataExtractedEvent and register the
  data_postprocessing_handler for this event. This adjusts the start and end
  date according to whole_day and open_end.

- Use default_timezone DatetimeWidget property. All datetime objects from
  plone.app.widgets' DatetimeWidget now have this timezone, if not otherwise
  set by the user.
declared min versions for p.a.widgets and z3c.form
dev checkouts
merge