Feature Request: Include an indicator before folders to visually display whether they are toggled or not

[zhenhua-wang]

Divirish has a nice folder state indicator in their line-prefix. I wonder if we could add a similar feature in dired-subtree as well.

[Fuco1]

Maybe it could be added to the fringe. But that would mean all the indicators would be lined-up at the edge rather than in front of the direcotry name. But due to the buffer structure of dired we can't add text in front of the directory name.