GTK needs to be built for CPDB-based printing

[tillkamppeter]

The GTK4 in the GNOME SDK Snap(s) needs to be built using the Common Print Dialog Backends (CPDB) to be able to print with the New Architecture based on IPP print services instead of print queues. The *.deb package of GTK4 is already switched over to CPDB and there you can see the needed changes (build configuration parameter).

This introduces a dependency on cpdb-libs which is not available in Ubuntu 22.04. Therefore cpdb-libs needs to get built from source by the Snap build of GNOME SDK, too. I have already released cpdb-libs 2.0b5 in the last days to fix an important bug concerning use in Snaps. This would be the version to be used currently (and by Snap release automation, the Snap should get auto-updated on new releases of cpdb-libs).

The communication between the CPDB-supporting print dialog and the print backends is done via D-Bus, so an appropriate interface needs to get plugged.

For applications which print via the included GTK3 there are 2 possibilities: First, one could try to backport GTK4's CPDB support to GTK3. Probably that would even not be too complex keeping in mind that the print dialog did not suffer many changes in all the time. The second, probably easier to implement way would be to make these apps using the print portal, so that they do not open a GTK3 print dialog at all.