KCupsConnection: Clean up CUPS connection/subscription
KCC is a QThread that provides a global for connecting to the CUPS notifier, KCC being the intermediary. At init (before the thread event loop is started), the dbus connections to CUPS is attempted for each signal and the "connected" list is then used to register the subscription with CUPS.
KCC uses QObject::connectNotify to internally register a successful dbus signal connect, so we're getting notified from the main thread and during this, we're starting our thread event loop. This is creating a timing issue such that the subscription renewal is thrashing and any signals that should emitted during this time are not registered in a timely manner, so not getting forwarded to the app's CUPS request.
I think this may have been done because earlier versions of the CUPS server were not performant or otherwise unreliable with the notifier/subscription.
In addition, a reliance on the (deprecated) older RH dbus events is present and plumbed upstream to the CUPS request and the individual apps. So they're actually all generally using both sets of signals for same notifier events.