Set parent window for dialogs
Summary:
This requires fix in Qt to work for standard dialogs:
https://codereview.qt-project.org/#/c/248559/
Reviewers: jgrulich, apol
Reviewed By: apol
Subscribers: graesslin, broulik, apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D17659