[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.

Test Plan:
Added a qDebug in ~ShadowHelper to ensure that _shadows is
empty.

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28073