Cleaner shutdown

The print-job manager was trying to recreate a notification service, so
destroy it earlier, and at the same time kill the content runner before
existing WebEngineContext::destroy() to catch any other late
recreations of singletons.

Change-Id: Ifaa333386248c3dbc6d4f828c727a0a17d5f5c90
Reviewed-by: Michael Brüning <michael.bruning@qt.io>