Add event_uid field, with index, to be used for ical sync

The ical spec requires a UID field to be set for syncing [1], the
recommended format is a UID specific to the host followed by "@" and
the host domain.

Note: this work was cancelled mid-flight because of a change in
requirements, it doesn't do much yet and is probably broken.

The idea was to set the event_uid on import. When syncing, an existing
event with the same event_uid could have it's fields updated to match
the current values in the ical feed.

1. http://www.kanzaki.com/docs/ical/uid.html
ical importer: update existing events, and only add new events when importing
Merge branch 'master' into ical-sync
ical importer: performance enhancement (thanks @thet)
Add catalog indexes in the setuphandlers instead of using GS

Thanks to @thet for pointing out that using the Generic Setup
catalog.xml would cause the index to be cleared every time the profile
is imported and for recommending this approach instead.
AT: add eventUid field for ical syncing
AT based test for #98, #97
pepsi
add missing changelogs
In portlet calendar and events, don't use the search_base directly to
  constuct calendar urls. The search base always starts from the Plone site
  root, which led to wrong urls in Lineage subsites.
pep8
- Remove the behaviors plone.app.relationfield.behavior.IRelatedItems adn
  plone.app.versioningbehavior.behaviors.IVersionable from the Dexterity
  example type. We don't depend on these packages and won't introduce an
  explicit dependency on it.
- Remove the behaviors plone.app.relationfield.behavior.IRelatedItems adn
  plone.app.versioningbehavior.behaviors.IVersionable from the Dexterity
  example type. We don't depend on these packages and won't introduce an
  explicit dependency on it.
not 4.4 but 5.0
not 4.4 but 5.0
- Restore Javascript based edit-form functionality to set end dates depending
  on start dates with the same delta of days as initialized, as developed by
  vincentfretin back at plone.app.event's birth.
Don't include the recurrence definition when doing icalendar exports of individual occurrences. Fixes: #61.
compactify docstrings
let this be more succinct, following seanupton suggestion
explicitly state relevant parameters