Fix mouse position handling for nested tty child frames

Fix mouse position handling for nested tty child frames

* src/dispnew.c (child_xy): New function.
* src/dispextern.h: Declare it.
* src/term.c (tty_frame_at): Return child-relative (x, y) in output
parameters.
(Ftty_frame_at): Return a list (FRAME CHILD-X CHILD-Y).
(handle_one_term_event): Adapt use of tty_frame_at.
* lisp/xt-mouse.el (xterm-mouse-event): Use new tty-frame-at.

(cherry picked from commit f37559a1ee035d184bc4db6e4f597281d3ac6879)