bug#28844: 26.0.90; display-line-numbers-mode should call window-configuration-change-hook

[Paul Rankin]

Activating/deactivating display-line-numbers-mode alters the window 
configuration and so should call window-configuration-change-hook but does not.

>From the Emacs NEWS file:

> Lisp programs that need to know how much screen estate is used up for
> line-number display in a window can use the new function
> 'line-number-display-width'.

This is nice, but in order to use this, we need to know when to act upon it, 
e.g. when display-line-numbers-mode affects the window configuration. Or, if 
there is a more appropriate hook.

To reproduce:

1. emacs -Q
2. add a hook to window-configuration-change-hook
3. M-x display-line-numbers-mode

Expected results:

Hook added should run when enabling display-line-numbers.

Actual results:

Hook is not run.

Configuration:

GNU Emacs 26.0.90 (build 1, x86_64-apple-darwin17.0.0) of 2017-10-15
macOS 10.13 (17A405)

MacBook Pro (Retina, 15-inch, Mid 2015)
2.2 GHz Intel Core i7
16 GB 1600 MHz DDR3
Intel Iris Pro 1536 MB