Printers: Make adminurl available for IPP printers

In the near future there will be an architectural change in CUPS. Printers will be IPP services published over DNS-SD (Avahi). Such printers will be configured over IPP and not over PPD which is deprecated. To have the most configuration options it would be good to have adminurl accessible in well accessible place. This link will get you to configuration page of the printer (typically run by the printer) where you can find options which are not even part of IPP standard.

There is something similar in "Printer Details" described as "Address" but I think that this could be removed for printers for which we will show the adminurl.

My idea is to have a new item in the combo box shown once user click on the settings icon for given printer which would be named for example "Administrative Web Page" or something like that right below "Printing Options" item (and maybe styled as link widget).

So there would be actually 2 ways how to configure the printer, one local for given computer (the "Printing Options") and second global for all users of the printer which would be more comprehensive.

Since there are such printers already shown in the Printers panel we can show the URL already for such printers and be ready once CUPS 3 is available (currently such printers show no options in the configuration under "Printing Options" though).

From technical point of view, we have to get this probably from "adminurl" txt record of the Avahi service since I don't see it as part of options I get from CUPS (because the local printer for the printer does not exist probably). See adminurl in "avahi-browse -vratp | grep "_ipp._tcp" | grep adminurl" for your network printer.

See #1877 for some other details.