Port to non-deprecated KWindowSystem::setMainWindow overload

Summary:
I wish there was a method like nativeWindow() which would take care of
both ensuring a native window and returning it, but well.

Test Plan: Builds

Reviewers: zzag, vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23215