Current GTK guidance says not to use gtk_file_chooser_set_current_folder.
Additionally, native file dialogs do not support previews on Windows, macOS, or when using portals. GTK4 does away with the API entirely.
Removing all this functionality means it's just a wrapper around creating the dialog, so just inline that and drop the function entirely.

Set an icon on a buddy, and also in a new account dialog.