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.
Hmm, so NemoCalendarManager dtor doing QThread::quit() omits the method invocation queue? Would be a slot, so wonder if it would be better doing invokeMethod. 

Alternatively can also add qWait back.