act-user-manager: Find the user's graphical session instead of assuming we are in it

If we were started by systemd --user, we won't be in a login session.
Find the user's graphical session by asking logind for it.

Closes: #42
1 job for master in 2 minutes and 15 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Test
passed #183405
build-fedora

00:02:15