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