Support GTK4

[Malizor]

For now only the gnome-chess module seems to use GTK4.
gladerunner.py does not accept its UI files in 4.0 mode as is, so some development may be needed.

[Malizor]

On Ubuntu 22.04 /usr/bin/broadwayd is provided exclusively by the libgtk-3-bin package, so I'm not sure it's possible to handle both GTK3 and GTK4 with the same Broadwayd binary…

Confirming that gtk4-demo cannot be launched with this version of broadwayd (contrary to gtk3-demo).
So first I need a way to install gtk4-broadwayd.

[Malizor]

Debian bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=994944
Ubuntu bug report: https://bugs.launchpad.net/ubuntu/+source/gtk4/+bug/1949299

[Malizor]

gtk4-broadwayd is now available in Debian sid and will be in Ubuntu 22.10.
I will play with it later when I have some time.