Occurrence listing changes

After merge, these changes will be forward-ported to master and 2.0 branches.

- Add an event_listing view for IEvent objects to show all of it's occurrences.
  [thet]

- Remove the visual distinction between IEvent and IOccurrences in the
  event_view. The user is likely not interested, if a Occurrence or the
  original Event is shown. Instead a Status Message is shown for users with
  rights to edit the event, saying that the original event, which can be
  modified, is one level up.
  [thet]

- Change the occurrence listing in the @@event_summary view to directly link
  to the occurrence objects, rename the label to 'All dates' and also include
  the first date of the original event.
  [thet]