Use hiddenInternal() for a ShellClient which got unmapped

Summary:
So far when a ShellClient got unmapped ready_for_painting was set to
false. That is the ShellClient was treated in the same way as a not
yet shown window. It was completely excluded from painting, a close
animation impossible.

This change makes use of the functionality available in
Client::hiddenInternal(). The window is considered as hidden, thus
still excluded from e.g. getting input events, but could be rendered
any time as we still have a previous window pixmap (if referenced).
This allows to have it considered in the rendering pass, but effects
still cannot make use of it as that state is not yet exposed to the
effects.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2083