Summary:
It doesn't make sense to pass const `QWindow`s to `PanelShadows` because
a `KWindowShadow` needs a mutable `QWindow` object.
Test Plan: const_cast in PanelShadows::Private::updateShadow is gone \o/
Reviewers: #plasma, apol
Reviewed By: apol
Subscribers: plasma-devel
Tags: #plasma
Differential Revision: https://phabricator.kde.org/D26985