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These changes also make use of touchpad and trackpad (Bug#26347).

* doc/emacs/frames.texi (Mouse Commands): Document horizontal
scrolling using the mouse wheel.

* lisp/mwheel.el (mwheel-scroll): Respond to wheel-right and wheel-left.
(mwheel-tilt-scroll-p, mwheel-flip-direction)
(mwheel-scroll-left-function, mwheel-scroll-right-function): New
defcustoms.
(mouse-wheel-left-event, mouse-wheel-right-event): New variables,
events that calls wheel-left/right.

* etc/NEWS: Mention horizontal scrolling using the mouse wheel.