bug#3142: 23.0.92; split-window-prefered-function should be able to use split-window-horizontally/split-window-vertically

[Juri Linkov]

> +   :type '(choice
> +       (const :tag "sensibly" split-window-sensibly)
> +       (const :tag "vertically"
> +              (lambda (window)
> +                (when (window--splittable-p window)
> +                  (with-selected-window window
> +                    (split-window-vertically)))))
> +       (const :tag "horizontally"
> +              (lambda (window)
> +                (when (window--splittable-p window t)
> +                  (with-selected-window window
> +                    (split-window-horizontally)))))

Using lambdas in defcustoms is too ugly: it will put this long lambda code
to custom-set-variables in .emacs, and cause other problems.

I suggest defining new functions with the prefix `split-window-preferred-'
that will provide a hint for the user where they are intended to be used:

split-window-preferred-sensibly (window)
split-window-preferred-vertically (window)
split-window-preferred-horizontally (window)

-- 
Juri Linkov
http://www.jurta.org/emacs/