Allow preconfiguration of domain, as given in authentication dialog under Username: domain\username

[Alex Myczko]

So I found https://github.com/OpenPrinting/system-config-printer/blob/master/authconn.py

I can see the AUTH_FIELD with 'domain', but that's not shown to me in the GUI, I'm able to
give the domain before the username with the syntax in the title of the issue.

Would there be a way to preconfigure the domain for all printers/users?