Also show mouse DND tooltip contents during interprogram drag-and-drop

Also show mouse DND tooltip contents during interprogram drag-and-drop

* doc/lispref/frames.texi (Drag and Drop): Document new
parameter to `x-begin-drag'.
* lisp/mouse.el (mouse-drag-and-drop-region): Don't hide tooltip
when initiating interprogram drag-and-drop.
* lisp/term/haiku-win.el (x-begin-drag):
* lisp/term/ns-win.el (x-begin-drag): Add stubs for new
parameter.
* src/xfns.c (Fx_begin_drag): New parameter `follow-tooltip'.
(Fx_show_tip, syms_of_xfns): Add records of the last dx and dy
given to `x-show-tip'.
* src/xterm.c (x_clear_dnd_monitors): New function.
(x_dnd_begin_drag_and_drop): Save monitor attributes list if
appropriate.
(x_dnd_compute_tip_xy, x_dnd_update_tooltip_position): New
function.
(x_dnd_update_state, handle_one_xevent): Update tooltip position
during DND mouse movement.
(syms_of_xterm): Update staticpros.
* src/xterm.h: Update prototypes.