gtk4-port: merge to master

GHex3 will be kept in maintenance mode until GNOME 42 at the least, but it is time to merge GHex4 in its pre-alpha state so that some translation can commence, and so that people can start testing it via the gnome-nightly flatpak repository.

It is unlikely anyone but myself will review and approve of this MR, but doing it as a MR will allow for some of the commits behind master to be reviewed, and see if there is any overlap for cherry-picking.