Don't use QTimer::callOnTimeout

Summary:
it is just a convenience function for connect that was added in Qt 5.12. Currently the minimum we require is 5.10
so just use connect.

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D21384