KWindowSystem: add setNewStartupId(QWindow *...) overload

Summary:
and deprecate the QWidget * overload, so that we can get rid of the
QtWidgets dependency one day.

Test Plan: Builds, new test passes

Reviewers: broulik, davidedmundson, zzag

Reviewed By: zzag

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23088