Emit calendar uid change signal when moving events between notebooks
Update cached notebook data correctly when notebooks are removed
Emit notebooksAboutToChange() signal before changing notebook data

The NemoCalendarManager::notebooksAboutToChange() signal is emitted
before cached notebook data is invalidated so that models depening
on the data can begin model reset at the proper time.
Begin model reset before notebook model is invalidated
Update comment in tst_calendarevent