The original dialog_inputbox doesn't work with longer than prompt_width
strings, here fixed it in this way:
1) add variable cursor_form_win to record cursor of form_win,
keep its value always between [0, prompt_width-1];
reuse the original cursor_position as cursor of the string result,
use (cursor_position-cursor_form_win) as begin offset to show part of
the string in form_win;