GTK4 has been released, we should eventually support it.

Most of the work has been done in #38 , but the major blocking issue is this. In summary, it is not possible to hide non-local files in the GTK4 file chooser (non-local files are those that do not have filepaths, such as resources on the Internet). We either have to filter the response and show an error message if non-local files are selected, or implement a shim to download those non-local files (not sure if this might be out of scope for NFD?).

Opinions on this issue are welcome.