bug#69305: outline-minor-mode for tabulated-list-mode

[Adam Porter]

By the way, with regard to grouping buffers specifically, this is what 
my Bufler package does, again in an extensible, user-programmable way:

https://github.com/alphapapa/bufler.el

I wrote it after putting significant effort into using Ibuffer's 
grouping features, which didn't meet my needs.

Bufler supports grouping buffers by many different attributes, such as 
directory, filename, mode, project, parent-project, TRAMP status, etc. 
And, significantly, it does such grouping dynamically at multiple 
levels.  For example, within a "~/src/emacs" directory, subgroups for 
git projects within that directory can be created dynamically, providing 
a grouping like:

+ Dir: ~/src/emacs
|-+ Project: ~/src/emacs/bufler.el
| |- *magit-status: bufler.el*
|-+ Project: ~/src/emacs/activities.el
  |- README.org

If org-super-agenda was my exploration of the topic, Bufler served as 
the prototype of a better API, which I then factored out into Taxy. 
Eventually I'll rebase Bufler on top of Taxy.