Move the precision pixel scrolling feature to pixel-scroll.el

Move the precision pixel scrolling feature to pixel-scroll.el

* etc/NEWS: Update NEWS entry for 'pixel-scroll-precision-mode'

* lisp/better-pixel-scroll.el: Remove file.

* src/pixel-scroll.el (x-coalesce-scroll-events): New variable
declaration.
(pixel-scroll-precision-mode-map): New variable.
(pixel-scroll-precision-scroll-down):
(pixel-scroll-precision-scroll-up):
(pixel-scroll-precision): New functions.
(pixel-scroll-precision-mode): New minor mode.