caldav: support more features for recurring events

[bitfehler]

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The protocol allows clients to ask the server to e.g. expand recurring events to a set of simple events in a given time-frame, so that the client does not have to calculate the recurrences.

This commit adds the foundation for support for expand 1, limit-recurrence-set 2, and limit-freebusy-set 3 functionality. However, the actual transformation of events returned to clients will have to be implemented in the backend, which now has the required information to do so.

[bitfehler]

This is not finished, but since this touches the exported API, can you let me know whether this approach makes sense to you?