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,
>>
>> that seems minor... one could always regenerate the index with
>> imenu-submenus-on-top set to nil when the export is requested?
>
> and forgot to add: in doc-view-mode we set already its local buffer
> value to nil (otherwise the generated outline makes no sense), so at
> least in one of the use cases that's never a problem.

Indeed, setting it locally to nil will solve this problem.