CalDAV workflows for Kronolith

Create/Update (davPutContents)

- rpc determines this is a dav request
- delegates to RPC dav backend
- Feeds into Sabre\Dav\Server
- SabreDAV stores the event to its calendar backend
- horde/dav Calendar backend internally finds out the caldav name of the calendar is served by kronolith
- Kronolith_Application::davPutObject is called.
- Resolve external calendar name to kronolith-internal calendar name
- Perform some access checks
- Parse icalendar data string into a Horde_Icalendar object
- Setup the Horde_Icalendar_Handler_Dav with the calendar backend and the icalendar

Kronolith_Icalendar_Dav (inherits Kronolith_Icalendar_Base)
- Get all components of the calendar
- ignore everything which is not a VEVENT
- try to load an existing kronolith event
- check if existing event last modified is newer / abort
- keep a list of exception events from the VEVENT for later
- Create a list of organizer and attendee(s) who signal by attribute that it's not the server's job to send them itip mails
- Create the new kronolith_Event base event object from the icalendar vevent
- Delete all kronolith_events which are exceptions on the base event from the backend
run the post-save actions for the base event
- check for missing mapping between external and internal event id, add if missing
- handle decision-making on itip: Is the change relevant for itip (prevent spam). Who should get an itip? which type of itip to send out?

back in the main process
- create an kronolith_event for each previously listed vevent exceptions and save it