Summary:
IncidenceChanger (optionally) contains a History which itself contains
an IncidenceChanger, so when disabling dialogs in the toplevel
IncidenceChanger, propagate that setting down to the inner
IncidenceChanger.

Test Plan: while ctest -R akonadi-sqlite-historytest ; do done

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D6868