Add some primitive momentum-based precision scrolling

Add some primitive momentum-based precision scrolling

The algorithm used to scroll the display kinetically is very
simple and needs improvement.  Someone should work on that
eventually.

* lisp/pixel-scroll.el (pixel-scroll-precision-use-momentum):
New user option.
(pixel-scroll-precision-mode-map): Add
`pixel-scroll-start-momentum'.
(pixel-scroll-kinetic-state):
(pixel-scroll-accumulate-velocity):
(pixel-scroll-calculate-velocity): New functions.
(pixel-scroll-start-momentum): New command.

* src/xterm.c (handle_one_xevent): Fix touch-end event
generation.