On X, avoid reporting unrepresentable touch IDs to Lisp

On X, avoid reporting unrepresentable touch IDs to Lisp

* src/xterm.c (xi_link_touch_point): Assign a Lisp-representable
identifier to the new touch point and return the same.
(xi_unlink_touch_point): New arg LOCAL_DETAIL.  Return such an
identifier if there is a matching touch point record.
(handle_one_xevent): Adjust as is proper.

* src/xterm.h (struct xi_touch_point_t) <local_detail>:
New field.  Reorder fields for alignment.

(cherry picked from commit b1692e23edc32ce8938d3af200c0c42c8aa6b313)