Popup$ (type, text, button)

   Synopsis:
      Opens a dialogue window and returns the user's response

   Notes:
      The Popup$ () function pauses execution to display a dialogue window, and
         resumes execution once the user clicks one of the window's buttons or
         closes the window manually.

      'text' is the text to use in the window.
      'type' specifies which icon is used in the window. It should be one of the
         following strings: "info", "warning", "error", "question".
      'button' specifies which buttons are used. It should be one of the
         following strings: "none", "ok", "close", "cancel", "yes-no",
         "ok-cancel".
      Underlines and spaces can be used instead of hyphens, so "ok-cancel",
         "ok_cancel" and "ok cancel" are all acceptable. However, the order of
         the words can't be changed (so the string "no-yes" won't be accepted).
      If you use invalid values for the type and/or response, default values are
         used instead, and no error is generated.

      Returns the button the user clicks (e.g. "yes" or "cancel"). If the user
         closed the dialogue window without clicking on a button, returns an
         empty string.

   Examples:
      ! Ask a question
      LET choice$ = Popup$ ("question", "Are you ready?", "yes-no")
      PRINT choice$
      END
