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.
Didn't see much need for these change checks: method already increases revision number so there's a change always. There's also a benefit of updating that + modification time even if no other changes happened. UI side can protect of unnecessary saves if there's reason.