tl,dr debian and ubuntu removed gksu, debian recommendation is to use "admin://"
https://bugs.launchpad.net/ubuntu/+source/umit/+bug/1740618
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=892768
This works ok un pcmanfm-qt but it does not in featherpad.
Upstream: FeatherPad issue 384