incidencedialog.cpp - ask to set a default events folder.

Summary:
When saving a new event, nag the user to set a default
calendar collection if they don't already have one.
Don't bother if they only have 1 events calendar folder.

{F8317182}

Test Plan:
- unset my default calendar.
- set my default calendar.
so far works as expected.

Reviewers: vkrause, dvratil, dfaure, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29725