code: public RFuture<Boolean> sysDialog (String title, String text, String ok, String cancel) {
      return RFuture.failure(new Exception("sysDialog not supported"));
    }
docs: Displays a system dialog with the specified title and text, an OK button and optionally a
  Cancel button.
  
  @param title the title for the dialog window. Note: some platforms (mainly mobile) do not
  display the title, so be sure your dialog makes sense if only {@code text} is showing.
  @param text the text of the dialog. The text will be wrapped by the underlying platform, but
  PlayN will do its utmost to ensure that newlines are honored by the platform in question so
  that hard line breaks and blank lines are reproduced correctly.
  @param ok the text of the button which will deliver a {@code true} result and be placed in
  "OK" position for the platform. Note: the HTML platform does not support customizing this
  label, so on that platform the label will be "OK". Yay for HTML5.
  @param cancel the text of the button that will deliver a {@code false} result and be placed in
  "Cancel" position. If {@code null} is supplied, the dialog will only have an OK button. Note:
  the HTML platform does not support customizing this label, so on that platform a non-null
  cancel string will result in the button reading "Cancel". Yay for HTML5.
  
  @return a future which delivers {@code true} or {@code false} when the user clicks the OK or
  cancel buttons respectively. If some unexpected error occurs displaying the dialog (unlikley),
  it will be reported by failing the future.