Provide native touchpad scrolling on macOS

Provide native touchpad scrolling on macOS

* etc/NEWS: Describe changes.
* lisp/term/ns-win.el (mouse-wheel-scroll-amount,
mouse-wheel-progressive-speed): Set to smarter values for macOS
touchpads.
* src/nsterm.m (emacsView::mouseDown): Use precise scrolling deltas to
calculate scrolling for touchpads and mouse wheels.
(syms_of_nsterm): Add variables 'ns-use-system-mwheel-acceleration',
'ns-touchpad-scroll-line-height' and 'ns-touchpad-use-momentum'.
* src/keyboard.c (make_lispy_event): Pass on .arg when relevant.
* src/termhooks.h (event_kind): Update comments re. WHEEL_EVENT.
* lisp/mwheel.el (mwheel-scroll): Use line count.
* lisp/subr.el (event-line-count): New function.