flatpak: install D-Bus .service on each frontend: work around lack of dependency handling in Flatpak by installing D-Bus .service on each frontend. This works because all backend is included in the runtime, but we have to add backend permissions to all frontend, and set --own-name=org.salutatoi.SAT. Furthermore, if one frontend is removed, the symbolic link is removed and the backend will not be launched automatically anymore, even if other frontends are still there. The benefict of this method is that backend has not to be installed manually to use a frontend.