Port zanshin to QDateTime, as used by the Applications/17.12 kcalcore API.

Summary:
Fixes compilation with Applications/17.12, should still compile
with earlier APIs.

Test Plan:
Compiles, tests pass (except for akonadi issues in storagetest but I'm
not running the 17.12 akonadi yet).

Reviewers: ervin, mlaurent, dvratil, vkrause

Reviewed By: ervin

Subscribers: KDE PIM

Differential Revision: https://phabricator.kde.org/D8817