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