[kstyle] Unregister a shadow when it's destroyed
Summary:
Since lifetime of a KWindowShadow doesn't strictly match the lifetime
of the associated widget, we need to unregister the shadow when it's
destroyed in order to prevent accessing or deleting dangling pointers
afterwards.
CCBUG: 416854
Reviewers: Plasma, broulik
Reviewed By: Plasma, broulik
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26967