Consider duplication prevention through deterministic UIDs (is it a good or bad idea?)

[tobixen]

Ref #29, when creating a new event, perhaps it could be an idea to make a deterministic uid based on a hash of the original icalendar object. This needs to be thought well through, perhaps google around and see if anyone else is doing things like this.

Benefits:

• The caldav server will prevent someone from accidentally add duplicated events.
• In some settings, maybe one may need to add the event to several calendar servers. It's probably a good thing to have the same event referenced by the same UID across different calendar servers.

Disadvantages:

• This may possibly prevent someone from intentionally add duplicated events
• This may possibly cause duplicated UIDs on independent icalendar objects created independently by different people, but with same timestamp and summary.