PanelView: defer deleting config view

This fixes a crash when the action is invoked from QML.

Object 0x55555a2e7fc0 destroyed while one of its QML signal handlers is in progress.
Most likely the object was deleted synchronously (use QObject::deleteLater() instead), or the application is running a nested event loop.
This behavior is NOT supported!
file:///usr/share/plasma/plasmoids/org.kde.plasma.private.systemtray/contents/ui/ConfigEntries.qml:290: function() { [native code] }

BUG: 476968 FIXED-IN: 6.0