Replace currentDateTimeUtc().toTime_t() with currentSecsSinceEpoch()

Summary:
toTime_t() is obsolete since Qt 5.8, and currentSecsSinceEpoch() is
faster.

Reviewers: Frameworks, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20163