Face-changing text properties and help-echo now work with xterm-mouse.

Face-changing text properties and help-echo now work with xterm-mouse.

* src/dispnew.c (update_mouse_position): New function for mouse
movement logic in 'handle_one_term_event' that can be shared across
different mouse backends.
(display--update-for-mouse-movement): New lisp function, call it.
* lisp/xt-mouse.el (xterm-mouse--handle-mouse-movement): New function
that calls 'display--update-for-mouse-movement'.
(xterm-mouse-translate-1): Call it.
* src/term.c (handle_one_term_event): Inline logic from
'term_mouse_movement' and call 'update_mouse_position'.
(term_mouse_movement): Delete.