Linux: add GTK4 support for Gnome Desktop 40+

[timlehr]

Currently we do not support GTK4 for our Linux builds.

I haven't tried building manually, but it looks like the cmake files indicate gtk3
I'm on Debian Trixie which is gtk4.

I'm not sure what distros like Rocky 9 use? But if they also provide gnome desktop in the 40+ range we should look into upping the gtk requirement.

Originally posted by @apetrynet in #133 (comment)

[jminor]

It looks like Raven isn't using GTK directly. Only the nativefiledialog submodule uses that. There's an open issue to address that here: mlabbe/nativefiledialog#113 but the project hasn't had commits for several years. Maybe we could switch to a different strategy for the File->Open dialog on Linux?

[jminor]

This draft PR onto a fork/enhanced version of nativefiledialog might be a way forward: btzy/nativefiledialog-extended#38

[jminor]

and another one, which might work around the issue? https://github.com/samhocevar/portable-file-dialogs