Backport MR67 to Gnome 45

This MR backports !67 (merged) into Gnome 45.

This patch ensures that xdg-desktop-portal-gnome works like both xdg-desktop-portal-gtk and xdg-desktop-portal-kde, translating any remote URI into a FUSE local file, before passing it to xdg-desktop-portal. This is required because xdg-desktop-portal disables, for security reasons, only understand file:// URIs.