Preparations for FileChooser implementation (Part 6 - Copy and Pasta🤌)

This is needed in order for nautilus to implement the portal itself, without having to adapt our widgets to run outside of the process of our NautilusApplication instance and without having to expose a public API for widgets to run for xdg-desktop-portal-gnome to use. Both of these would have been a bigger maintenance burden than copying and pasting some code which is not expected to change anyway.

See early discussion at https://discourse.gnome.org/t/planning-filechooser-portal-implementation-with-nautilus/20335/4

Part of #3401 (closed)