Improve handling of pointer focus under the input extension

Improve handling of pointer focus under the input extension

* src/xfns.c (x_relative_mouse_position)
(Fx_mouse_absolute_pixel_position, compute_tip_xy): Use
x_query_pointer instead of XQueryPointer.
* src/xterm.c (xi_populate_device_from_info): Set new attachment
field.
(xi_handle_focus_change): Set client pointer.
(xi_focus_handle_for_device, xi_handle_interaction): Use
attached keyboard device to handle focus.
(x_query_pointer): New function.
(XTmouse_position, x_scroll_bar_report_motion)
(x_horizontal_scroll_bar_report_motion, handle_one_xevent): Use
x_query_pointer instead of XQueryPointer.
(x_term_init): Initialize client pointer device.
* src/xterm.h (struct xi_device_t): New field `attachment'.
(struct x_display_info): New field `client_pointer_device'.