bug#51210: Customizable other-window-for-scrolling

[martin rudalics]

> Maybe like there is a variable other-window-scroll-buffer,
> it would be nice to add a new user option
> other-window-for-scrolling-function that will return
> a window for scrolling.  It could provide a choise list that
> includes a function that returns the most-recently-used window.

Maybe a variable 'other-window-scroll-window' whose value could be a
function to get that window.

martin