Make `idle-update-delay` obsolete

Make `idle-update-delay` obsolete

This user option was introduced in 2003, and was advertised as a general
variable for a "delay before updating various things on the screen".
But this has never been true: it was only used by 'which-function-mode',
and, to make matters worse, users of that mode would basically never
know that this user option existed without reading the code.

Conversely, users that did find the user option would be surprised to
see that it only took effect in 'which-func-mode'.

The lack of other users of 'idle-update-delay' reveals that it has not
been considered generally useful.  Thus, it makes more sense to
introduce a new mode specific user option `which-func-update-delay`, and
to mark the old one obsolete.

* lisp/simple.el (idle-update-delay): Make obsolete.
* lisp/progmodes/which-func.el (which-func-update-delay): New defcustom.
(which-function-mode): Use above new defcustom instead of
'idle-update-delay'.

(cherry picked from commit a212687e24fb6a7492db28e62070b03b43784660)