Suggestion: When available, prefer using org.freedesktop.portal.FileChooser in FileChooserNative

Title. Currently usage of the XDG Desktop Portal FileChooser API is limited to Flatpak but removing that restriction would better fit the goals of FileChooserNative since it would allow Gtk applications to integrate better with the user's desktop on Linux just as well as it does on macOS/Windows.

It would also solve, in a way, longstanding issue #233 (closed) by letting desktops handle it without increasing Gtk complexity and provides a reasonable solution to the most common reason for users abusing GTK_USE_PORTAL (related: !4829 (merged)).