build: add GNOME GitLab CI configuration
The pipeline begins with "build" stage doing a distcheck on Fedora 28 (which is still known to ship libnm-glib) and outputting a tarball artifact. The output is then used in the "test" stage. In future, builds on some older platforms, CentOS and Ubuntu and clang builds would be nice. Not implemented at this point, but it should be straightforward enough. Maybe a build with a Git snapshot of NetworkManager and/or network-manager-applet would be useful at some point, but that's not implemented either.