Call for Tests: iCalendar offset and thing split

As openHAB 4 is now released, it would be nice if some users could test this Pull request (built binary for openHAB 4 linked there, take the version 4.1.0-SNAPSHOT).

Just make sure to uninstall a previously installed version. And please do a backup to be sure, as things will be upgraded and there is no other way back.

As things have been changed, please check out the updated readme:

Thanks to @RRoe for already doing a first test. Please report bugs or findings directly in the PR. This topic is more for discussion.