Allow falling back to other portals

With xdg-desktop-portal 1.18+, e.g. https://github.com/flatpak/flatpak/issues/2861 is back even on systems w/ xdg-desktop-portal-gtk because only the kde portal is used for settings. Even if we implement that particular nonstandard key, people might be relying on xdg-desktop-portal-gtk for exposing other gsettings to gtk applications in flatpaks.

This MR also allows flatpak applications that use libsecret without org.freedesktop.secrets access to continue working, since otherwise that interface will not be exposed even on systems with gnome-keyring installed, as it's not implemented by -kde.