Implement `follow-tooltip' for DND on Haiku

Implement `follow-tooltip' for DND on Haiku

* lisp/term/haiku-win.el (x-begin-drag): Implement
`follow-tooltip'.
* src/haikufns.c (Fx_show_tip): Record last dx and dy.
(syms_of_haikufns): New staticpros.

* src/haikuselect.c (haiku_unwind_drag_message): Clear new flag.
(Fhaiku_drag_message): New argument `follow-tooltip'.  Set new
flag.
(haiku_dnd_compute_tip_xy): New function.
(haiku_note_drag_motion): Move tooltip if flag is true.

* src/haikuterm.c (haiku_read_socket): Don't generate help event
if mouse moves onto a tooltip during DND.
* src/haikuterm.h: Update prototypes.