(split-window-sensibly): New function.

(split-window-sensibly): New function.
(split-height-threshold, split-width-threshold): State in
doc-string that these affect split-window-sensibly.  Change
customization subtype from number to integer.
(window--splittable-p): Rename to window-splittable-p since it's
referred to in doc-string of split-window-sensibly.  Update
doc-string.
(window--try-to-split-window): Unconditionally call
split-window-preferred-function and move splitting functionality
to split-window-sensibly (Bug#3142).
(split-window-preferred-function): Rewrite doc-string.  Don't
allow nil as customization type.