Fix panelview config cleanup

Summary:
Connecting to QObject::destroyed with Qt::QueuedConnection *should*
crash, when you use the object that's been deleted. it doesn't make any
logical sense.

The only reason it doesn't is because of another hack in Plasma-
framework where containment.cpp where containment emits
QObject::destroyed (yes really) before the actual destruction.

Unfortunately we can't fix the hack in frameworks till we fix this hack
round the original hack here.

This patch ports it to a different signal, which is emitted before
object teardown without hacks.

Test Plan: checked debug appeared when deleting a panel

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2570