Use QQC2 Dialog
Summary: Replace custom dialog with QQC2 Dialog
Test Plan:
Reviewers: #calligra:_3.0, leinir
Reviewed By: leinir
Subscribers: danders, Calligra-Devel-list
Tags: #calligra:_3.0
Differential Revision: https://phabricator.kde.org/D26221