Expose calendar-uri as property of deleted caldav events

The trashbin is one report across all calendars of a user. In order to
refresh the data of a single calendar when one of its events is
restored, we need to know what calendar the event belongs to. There
doesn't seem to be a standard property for a URI, so this adds a custom
Nextcloud prop for the calendar-uri.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>