[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