bug#43568: Horizontal mouse wheel scrolling

[Juri Linkov]

> Currently the mouse-wheel horizontal scrolling step is hard-coded to the
> constant 1.  It's very inconvenient to scroll large amount of text
> horizontally by small increments.  For example, the horizontal step is
> hard-coded to 3 in Firefox, and to 5 in Chrome.
>
> But since Emacs is much more powerful than web browsers in regard to
> customization, the following patch adds a new user option to define the
> default step, and also allows the user to change the step dynamically by
> using a numeric prefix arg before starting to scroll, e.g. typing 'M-8'
> before scrolling will set the scroll step to 8.

Pushed now.