In follow mode windows in a GUI, don't display inactive cursors

In follow mode windows in a GUI, don't display inactive cursors

This is done by setting cursor-in-non-selected-windows buffer locally.

* lisp/follow.el (follow-hide-ghost-cursors): New customizable option.
(follow-mode): Create and set, or kill buffer-local copy of
cursor-in-non-selected-windows when the mode gets enabled or disabled.
(follow-prev-buffer): New variable.
(follow-adjust-window): Manipulate cursor-in-non-selected-windows when the
current buffer changes, to ensure that cursors stay visible in non-selected
follow window groups.

* etc/NEWS: Add an entry for this change.