bug#52293: 29.0.50; [PATCH] Prevent further cases of duplicated separators in context menus

[Jim Porter]

This is a followup to bug#52237. I'll just quote my message describing 
the issue from there[1]:

> I found another odd case, but I'm not 100% sure the best way to fix it:
>
>   emacs -Q --eval '(context-menu-mode)'
>   C-h o identity RET
>   ;; Right-click somewhere in the Help buffer
>
> There's a doubled separator after "Next Topic". Looking at the code, this is 
> because `help-mode-context-menu' inserts new items using `define-key', which has the 
> effect of putting the new items *before* the (hidden) menu title. The resulting keymap 
> ends up looking like this:
>
>   (keymap
>    (Previous\ Topic ...)
>    (Next\ Topic ...)
>    (help-mode-separator "--")
>    #("Context Menu" 0 12 (hide t))
>    (separator-undo "--")
>    ...)
>
> Since there's a hidden item (the keymap title) between the 
> `help-mode-separator' and `separator-undo'[1], the de-duplication doesn't 
> handle that.

Attached is a patch to fix this based on the discussion in bug#52237. 
One slightly odd thing is that for context menu functions that put their 
items at the top, they place their separator *below* the items. Other 
functions place the separator *above* the items. It might be too late to 
fix this though, given that Emacs 28 is only open for fixing 
regressions, and changing it in 29 would be a (small) compatibility 
break. However, I can update the patch to put the separators in these 
functions at the top if people think that's best.

[1] https://lists.gnu.org/archive/html/bug-gnu-emacs/2021-12/msg00143.html

0001-Add-a-top-separator-to-the-context-menu.patch
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