kcm: Handle the CUPS service being unavailable

Currently, if the CUPS service fails or is not started properly, the KCM provides a message that there are no printers currently configured. While potentially true, this does not give meaning info for the user about what's really going on.

Add a placeholder message to provide some meaningful options in case the CUPS service is unavailable. Also, modify the "empty devices" placeholder to be a loader item. This makes use of already-existing signals from the PrinterModel when it attempts to load.

Testing

Stop the CUPS service and run the kcm, the "no service" placeholder should appear. (Re)start the cups services and it should auto-load configured printers.

Removing all printer devices should display the existing "No printers...." placeholder.