[tableview] Add some unique element to AbstractColumnRenderer.__str__ (related to #3351872) Otherwise different instances look the same which is confusing.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances