Backport from seanupton: IObjectModifiedEvent subscriber returns early on
  newly created event (Commite c60c8b521c6b1ca219bfeaddb08e26605707e17 on
  https://github.com/seanupton/plone.app.event).
fix ical importer to work properly with DX and AT objects. since they have different fieldnames and thus different invokeFactory signatures, we better set start,end,timezone and whole_day via the accessor afterwards.
rm unecessary DT wrapping. create_ method stub
better stub
todo