Summary:
QApplication::quit will close any open windows including error messages.

This keeps the process alive whilst we have a top level window.

Test Plan:
Ran a kioclient script that produced an error dialog
It stayed on my screen

Reviewers: #plasma, nerdopolist, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17653