Add addToplevel and removeToplevel to Scene

Summary:
Code that destroys all scene windows when finishing compositing is not
readable. E.g. can you tell what this piece of code is doing without
looking into the source code of Scene::windowClosed?

    foreach (Client * c, Workspace::self()->clientList())
        m_scene->windowClosed(c, NULL);

This change intoduces removeToplevel(as well its counterpart) method to
the Scene class. The name of the new method much better describes what
we're doing.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18210