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

[Juri Linkov]

>>> 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
>>   ...
>
> Creating an outline-mode buffer like this is easy to do, but
> I doubt that a duplicate of imenu-list.el is needed to be in core,
> because I'm sure in this case people will ask to copy more
> features from imenu-list.el such as imenu-list-auto-update, etc.

With the imenu-flatten option 'group' proposed in bug#70846
it will be possible to use exactly the same outlines as above
in the Completions buffer.