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

[Adam Porter]

> > Data structure for `tabulated-list-groups' is a list of elements,
> > each element should of the form
> >
> >   (GROUP-NAME (ID . CONTENTS) (ID . CONTENTS) ...)
> >
> > where GROUP-NAME is the name of the group, ID is a Lisp object that
> > identifies the entry and CONTENTS is a vector with the same number of
> > elements as `tabulated-list-format'.  When buffers grouped by major mode:
> >
> >   (("* Lisp Interaction" (#<buffer *scratch*> ["." " " "*" #("*scratch*" 0 9 ...) "225" "Lisp 
> > Interaction" ""]) ...))
>
> This is the value that the user is supposed to supply for the
> defcustom?  That is, the user must specify all the buffers explicitly?
> What happens when there's one more buffer in some mode? does the user
> have then to modify the value of the defcustom?

FWIW, using, e.g. Bufler/Taxy to group buffers, the user would define a 
set of group key functions like so:

  (setf buffer-keys '((project) (special mode) mode))

That in effect says, "If a buffer is part of a project, group it by that 
project's directory; otherwise if a buffer is special, group it with 
other special buffers, and create subgroups by mode; otherwise group it 
by mode."

Then the groups are created dynamically at runtime when the buffer list 
buffer is refreshed.