Create/check for our unity first-run flag

- Add a Python script via an XDG autostart file to watch
  for the end of the session, and set a flag file if we have
  run Unity.
- Check for our new unity first-run flag and pre-populate
  $XDG_CACHE_HOME/unity with a first-run stamp file, if we've
  already run Unity before.
  Workaround for LP: #1328677 and (Trac: #1481)