Add support for touchscreen events generated by XInput 2

Add support for touchscreen events generated by XInput 2

* doc/lispref/commands.texi (Input Events): Add "Touchscreen
Events" to menu.
(Touchscreen Events): New node.
* etc/NEWS: Announce new event types.

* src/keyboard.c (make_lispy_event): Handle touchscreen events.
(syms_of_keyboard): New symbols `touchscreen-begin',
`touchscreen-end' and `touchscreen-update'.
* src/termhooks.h (enum event_kind): New touchscreen events.

* src/xfns.c (setup_xi_event_mask): Set up touch event mask if
appropriate.
(x_window) [USE_GTK]: Call `setup_xi_event_mask'.

* src/xterm.c (x_free_xi_devices): Free touchpoints.
(x_init_master_valuators): Handle all devices and set master and
touch mode flags appropriately.
(xi_reset_scroll_valuators_for_device_id):
(x_get_scroll_valuator_delta): Ignore slave devices.

(xi_link_touch_point):
(xi_unlink_touch_point):
(xi_find_touch_point): New functions.
(handle_one_xevent): Handle XI touch events and events from
non-master devices.

* src/xterm.h (struct xi_touch_point_t): New structure.
(struct xi_device_t): Add touchpoints field.