(IT_note_mode_line_highlight, IT_note_mouse_highlight):

(IT_note_mode_line_highlight, IT_note_mouse_highlight):
Record the object that generated the help echo and the position of
that object in help_echo_object and help_echo_pos.  Record that
some glyphs in a row are displayed in mouse-face.
(IT_update_begin): Don't clear mouse highlight unless one of the
enabled glyph rows is marked as being displayed in mouse-face.
(dos_rawgetc): Generate 2 events for HELP_EVENT.  Pass the object
and position recorded in help_echo_object and help_echo_pos to the
event queue.
(IT_menu_display): Accept a new argument PN: the pane number of
the current menu pane.  Record the pane number and the item
number of the item which has associated help string.
(XMenuActivate): Update the prototype for help_callback in
function declaration.  Call IT_menu_display with the current menu
pane number as an additional argument.  Call help_callback with
two additional arguments: the pane number and the item number of
the menu item associated with the help text.
(help_echo_object, help_echo_pos): New variables.
(syms_of_msdos): Initialize them and staticpro help_echo_object.
(help_echo_window): New variable.
(syms_of_msdos): Initialize and staticpro it.
(IT_note_mode_line_highlight): Set help_echo_window.
(IT_note_mouse_highlight): Ditto.
(dos_rawgetc): Store help_echo_window in the second event produced
for HELP_EVENTs.