Add KWindowSystem::setMainWindow(QWindow *) overload

Summary: All QWidget-based API is now deprecated in this module.

Test Plan: Builds.

Reviewers: broulik, davidedmundson, zzag

Reviewed By: zzag

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23108