bug#26347: patch for mwheel.el

[Tak Kunihiro]

Please revise words and phrases as you wish.  On info revision, I also
put a recipe to scroll less.  Please discard the part if that should
be done by different process.

# ChangeLog

Support scrolling by tiling wheel

Scroll right and left using wheel-right and wheel-left; This revision
also makes use of touchpad and trackpad (Bug#26347).
* doc/emacs/frames.texi (Mouse Commands): Document the change and recipe of 
scroll less
* lisp/mwheel.el (mwheel-scroll): Respond to wheel-right and wheel-left
* lisp/mwheel.el (mwheel-tilt-scroll-p): Enable tilt scrolling
* lisp/mwheel.el (mwheel-flip-direction): Flip scrolling direction
* lisp/mwheel.el (mwheel-scroll-left-function): Function to be called by 
wheel-left
* lisp/mwheel.el (mwheel-scroll-right-function): Function to be called by 
wheel-right
* lisp/mwheel.el (mouse-wheel-left-event): Event that calls wheel-left
* lisp/mwheel.el (mouse-wheel-right-event): Event that calls wheel-right

# NEWS

** Emacs can scroll horizontally using mouse, touchpad, and trackbar.
You can start scrolling by customizing `mwheel-tilt-scroll-p'.  When
direction of scroll is opposite, customize `mwheel-flip-direction'.

# Info

--- doc/emacs/frames.texi 2017-04-12 08:09:52.565691400 +0900
+++ doc/emacs/frames-b.texi 2017-04-12 08:16:45.075204400 +0900
@@ -206,6 +206,22 @@
 @code{mouse-wheel-progressive-speed} determines whether the scroll
 speed is linked to how fast you move the wheel.

+Emacs also supports horizontal scrolling by tilting ``wheel''.  The
+variables @code{mwheel-tilt-scroll-p} turns the feature on.  When
+direction is opposite as you wish, turn the variable
+@code{mwheel-flip-direction} on.
+
+Occasionally scrolling by a ``wheel'' is accelerated by system.  In
+such case, default configuration may scroll too fast.  If so, for
+example, you can add the following lines to your init file
+(@pxref{Init File}) to scroll less.
+
+@example
+(setq mouse-wheel-scroll-amount '(1 ((shift) . 5) ((control))))
+(setq mouse-wheel-progressive-speed nil)
+@end example
+
 @node Word and Line Mouse
 @section Mouse Commands for Words and Lines