Changes for supporting mouse/modeline highlight and help echo features:

Changes for supporting mouse/modeline highlight and help echo features:

(dos_set_window_size) [__DJGPP__ > 1]: If the frame
dimensions changed, invalidate the mouse highlight info.
(disable_mouse_highlight, help_echo, previous_help_echo): New
variables.
(IT_set_mouse_pointer, show_mouse_face, clear_mouse_face)
(fast_find_position, IT_note_mode_line_highlight)
(IT_note_mouse_highlight): New functions.
(IT_update_begin): If the redisplay affects the window where the
mouse highlight is, clear the highlight.  If the frame where the
highlight was displayed was killed, invalidate the highlight
info.
(IT_update_end): Reset the highlight flag.  Reset the mouse
highlight-defer flag.
(IT_frame_up_to_date): New function, if mouse highlight was
deferred due to GC, do it now.
(internal_terminal_init): Initialize mouse-highlight related
members of the_only_x_display.  Assign IT_frame_up_to_date to
frame_up_to_date_hook.
(dos_rawgetc): If the mouse moved, update mouse highlight.  If
help_echo changed value, generate a HELP_EVENT event.
(syms_of_msdos): Staticpro help_echo and previous_help_echo.