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

[Juri Linkov]

>> 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
>> ...
>
> That's a good point.  Perhaps we need to create a special mode to
> display the imenu hierarchy.

This problem occurs only when MENU-TITLE is nil in imenu-generic-expression.
We could invent a fictitious outline heading for such empty titles, e.g.
"All" or "Other".  Then it will have own outline:

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