[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