examples: Add systemd startup script and example session

This allows running a session using a separate systemd user instance.
The advantage is full separation of the normal user (e.g. to run a
nested session) and generally a better flexibility and more
straight-forward way of pulling in systemd units from jhbuild.
2 jobs for wip/benzea/systemd-user-session in 14 minutes and 40 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed test-py3 #831446

00:03:37

passed ub19.04-py3-glib #831447

00:14:39