Correctly handle touches on the tool bar

Correctly handle touches on the tool bar

* src/xterm.c (xi_link_touch_point): New argument `frame'.  Set
new field `touchpoint->frame' to it.
(xi_unlink_touch_points): New function.
(xi_disable_devices): Clear the tool bar device on frames whose
tool bar device matches this field.
(handle_one_xevent): If an XI_TouchBegin event lands on the tool
bar, then simulate a pointer click.  Ignore future events from
that touchpoint from there onwards.
(x_make_frame_invisible, x_free_frame_resources): Unlink touch
points associated with the frame.

* src/xterm.h (struct xi_touch_point_t): New field `frame'.
(struct x_output): New fields for keeping track of tool bar
touches.