Keep internal clients in the unmanaged layer

Summary:
All internal clients live in the x stacking order, but when such a
client is closed, it will be moved to the normal stacking order.

Given that internal clients don't specify the desired layer, they will
be moved to the normal layer, which is not really what we want because
it means that the task switcher window will be placed below docks.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D21116