CalDAV workflows for Kronolith

Create/Update (davPutObject)

- 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

TODOs/Refactoring

TODO: Factor out the iTip handling into a separate service which just takes previous and current event and possibly some intent control
TODO: Make this flow unit testable (How?)
TODO: Make the iTip flow reusable for web UI and inter-app API cases.

Ajax saveEvent vs the davPutObject flow

- UI has a different data/attribute format
- more context or intent can be communicated ("as new", "send updates/no updates", "move event to different calendar"
- more specific permission checks
- Add event can check for a limit
- edit event can check for edit permissions
- very similar itip related code