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

[Juri Linkov]

Often scroll-other-window uses a wrong window for scrolling
when there are more than 2 windows on the frame.

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.

Then maybe also minibuffer-handling logic could be moved from
other-window-for-scrolling to the function used by default
in this new option.