Call `system-idle-seconds' to get the number of seconds since last user
activity on the computer.

This differs from the built-in `current-idle-seconds', which can only be
used for that purpose as long as Emacs is "in focus".

This differs from `org-user-idle-seconds' in org-clock.el, by adding
support for Wayland.  Hopefully more in the future.  Plus your code won't
have to depend on loading Org, just this little library.

Supported environments:

- Mac OS
- Wayland (GNOME)
- Wayland (KDE and others supporting ext-idle-notify)
  - https://wayland.app/protocols/ext-idle-notify-v1#compositor-support
  - Returns 0 if invoked in the first 9 seconds or so
  - Requires installing "swayidle"
- X11 (GNOME)
- X11 (any)
  - Requires installing "x11idle" or "xprintidle"

To test that it works, eval the following and do not touch the computer for
11 seconds.  A number should be printed to *Messages*.

    (run-with-timer 11 nil (lambda () (print (system-idle-seconds))))

A known issue: `system-idle-seconds' is not guaranteed to return correct
results if user has switched from the graphical desktop to a TTY console
and is in the console by the time it is called.