Return the total height, in lines, of window WINDOW.
WINDOW must be a valid window and defaults to the selected one.
The return value includes the mode line and header line, if any.
If WINDOW is an internal window, the total height is the height
of the screen areas spanned by its children.
On a graphical display, this total height is reported as an
integer multiple of the default character height.
(fn &optional WINDOW)