[Panel Shadows] Un-constify QWindow parameters

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