[44] Expose settings backend when running outside of GNOME

This means it'll no longer exit when launched in an incompatible environment, but will continue but only exposing the org.freedesktop.impl.portal.Settings interface, and none of the other portals.

This is needed to fulfill the expectations of xdg-desktop-portal, who will unconditionally launch every portal backend and merge all the settings from each and every portal backend into a single look up table.

Fetching the session bus is moved earlier, as otherwise it might try to reuse the failed attempt from trying to initialize GTK.