[KJots] Make compiler happier (use overrides, nullptr, explicit, QT_NO_KEYWORDS)

Summary:
This patch does not contain any functional change, it only does following:

1. Add override keyword where applicable (also, remove empty dtors);
2. Make ctors explicit;
3. Use nullptr instead of 0;
4. Add QT_NO_KEYWORDS, make code compile

Test Plan: it compiles

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28993