Fix adding new printers

Now that we've fixed discovering printers !3846 (merged), let's handle adding them correctly too. While IPP discoverable printers show up immediately in g-c-c, adding others i.e. USB ones should be fixed by this change (I only have IPP discoverable for testing, but one can add it manually triggering the same bug).

Moreover, the printing itself doesn't work without the following change in freedesktop-sdk, so backport it after it's merged: https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/merge_requests/27763

Fixes: #1066 (closed)