Printing / CUPS support

[bonanza123]

https://github.com/flatpak/flatpak/releases/tag/1.6.0 mentions that --socket=cups is needed to have CUPS printing support. Would it possible to add this?

[refi64]

Even with that printing doesn't work 🤔 I'm not sure why...

[bonanza123]

seems to be a problem with flatpak itself. Hopfully it will be fixed in a future version. flatpak/flatpak#3355

[yajo]

I remember to have been able to print with MasterPDF in the past. But now it can't find the printers. Any clues on what happened? (Maybe I had some overrides with flatseal that I forgot in this new PC?) Is somebody able to print with it?

[muelli]

adding CUPS to the sockets: #84

[mmattel]

Closing because it got fixed via #84 ?