add-printer: Minor clean up and exiting fix

See commits. Add categorized logging and a deprecation message.

The current app does not exit properly after a printer is added. The widget disappears, but the app remains running. So quit the app if the user clicks "Finish" at the final step.

There is no bug report and this would be a rare use case given the kcm now handles this. Found while testing various configs with and without the system-config-printer helpers.