[kstyle] Properly unregister widgets in ShadowHelper

Summary:
After a widget has been unregistered, a WinIdChange event may be sent.
If that happens, ShadowHelper is going to try to install a shadow for
the corresponding widget. Obviously, this is wrong.

Reviewers: #plasma, cblack

Reviewed By: cblack

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28075