[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.
BUG: 416854
Test Plan: plasmashell no longer crashes.
Reviewers: Plasma, broulik
Subscribers: apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26966