bug#58532: 29.0.50; Support displaying imenu entries in a separate buffer

[Juri Linkov]

> It'd be great if imenu supported displaying its alist of entries in a
> separate buffer.  This is particularly useful when the list of entries
> has many levels (for example, it is the table of contents of a PDF,

Unfortunately, mupdf/mutool doesn't support Unicode characters:
https://bugs.ghostscript.com/show_bug.cgi?id=702358
Maybe doc-view--pdf-outline could post-process the output
to replace hex-encoded numbers with their character equivalents?

> supported in Emacs 29), because you could search for an entry very
> easily.  Right now, pressing C-s in the minibuffer only searches for
> first-level imenu entries.
>
> I suggest the new buffer would be configured in outline-mode, which is
> suited for this kind of hierarchical content.

The problem is that imenu-submenus-on-top is t by default,
but such order is incompatible with the outline-mode trees.
For example, all functions belong to the Variables outline:

* Types
  imenu
  imenu-unavailable
* Variables
  imenu-after-jump-hook
  imenu-auto-rescan
  imenu-auto-rescan-maxout
  ...
imenu
imenu--cleanup
imenu--completion-buffer
imenu--create-keymap
...