Add CalDAV interface

[mhthies]

The CalDAV interface should be able to read a specific calendar from a CalDAV calendar server. It should allow to create Readable + Subscribable bool-typed connector objects that tell whether currently any event, matching a specific pattern, is running according to the calendar. The connector-specific patterns could allow to consider only busy (not free) events, or only events having their summary or description matching a specific string pattern.

In addition, the interface could allow to create similar connectors that return the datetime timestamp of the next matching event.