Show missing soft dependencies
We need to tell the users when a soft dep is missing, so they don't blame us. Our flatpak build will always have all the deps, but the packages made for various distros could be missing deps.
We might want to show an infobar in the greeter, and we might want to use the dialog removed in 2dfa9de9 .. or something simpler if possible. In any case it won't be called "Manager" anymore.