Create separate class for making event modifications

WIP: should update unit tests before merging. Created pull request already for feedback.

Separate class simplifies the manager code a bit and should allow later better to make modifications else than main even, i.e. adding exceptions to recurring events. See commits for details.
No signaling, no way to know about removal done unless e.g. event query gets added. Would seem overkill here, currently it's removing only one instance. Actually, the waiting doesn't even have any effect until more events are to be removed.