Expose the ``cancellation'' of touch events to Lisp

Expose the ``cancellation'' of touch events to Lisp

* doc/lispref/commands.texi (Touchscreen Events):
* etc/NEWS: Describe new event parameter `canceled'.
* src/keyboard.c (make_lispy_event) <TOUCHSCREEN_END_EVENT>:
If event->modifiers, set canceled to t.

* src/termhooks.h (enum event_kind): Describe meaning of
modifiers in TOUCHSCREEN_END_EVENTs.
* src/xfns.c (setup_xi_event_mask): Select for
XI_TouchOwnership.
* src/xterm.c (xi_link_touch_point): Round X and Y and clear
ownership.
(xi_unlink_touch_point): Return 1 if the touch point is not
owned by Emacs, 2 otherwise.
(handle_one_xevent): Handle XI_TouchOwnership events and report
ownership correctly.
* src/xterm.h (enum xi_touch_ownership): New enum.  Write
commentary on XI touch sequence ownership.
(struct xi_touch_point_t): Use integer X and Y.  New field
`ownership'.  Adjust for alignment.