[App Style KCM] Use QtQuick NewStuff dialogs
Summary:
Make the dialogs consistent with others KCMs by using QtQuick versions.
Test Plan: Test dialogs for GTK themes installation.
Reviewers: broulik, ngraham
Reviewed By: ngraham
Subscribers: jferrer, Localization, yurchor, ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26717