Return the total width, in columns, of window WINDOW.
WINDOW must be a valid window and defaults to the selected one.
The return value includes any vertical dividers or scroll bars
belonging to WINDOW. If WINDOW is an internal window, the total width
is the width of the screen areas spanned by its children.
On a graphical display, this total width is reported as an
integer multiple of the default character width.
(fn &optional WINDOW)