Prompts for confirmation asking a cancel/continue style question.
Set the various properties on the prompt before calling this function to represent the question correctly.
This method will block until the a response is returned from the prompter and will run a main loop similar to a `gtk_dialog_run()`. The application will remain responsive but care must be taken to handle reentrancy issues.
gcr_prompt_reply_continue will be returned if the user confirms the prompt. The return value will also be gcr_prompt_reply_cancel if the user cancels or if an error occurs. Check the throws argument to tell the difference.
optional cancellation object
the reply from the prompt