bug#9054: 24.0.50; show source in other window

[Juri Linkov]

>> The back-end should return collected data as alist,
>> and front-end should render data as the buffer text.
>
> Hmmm...  I still don't get it.

For example, ‘C-h f’ (describe-function) first calls a back-end function
that collects all information about a function and returns it in a such form:

  ((type . function)
   (name . "car")
   (source-file . "src/data.c")
   (Info-node . "(elisp) List Elements")
   (docstring . ...)
   ...)

Then it gives this data to a front-end function that will render it
as buttonized text in the Help buffer, or as html in a eww buffer,
or as Info markup in an Info buffer, etc.

>> Indeed, now there are such modifiers: ‘C-x 4 0’ and ‘C-x 4 4’.
>
> I meant modifiers for mouse-1 clicks on the menu entry.

But ‘C-x 4 0’ and ‘C-x 4 4’ already do what is needed,
and indeed I tested these work fine: ‘C-x 4 0 down-mouse-3’.