The ai_response_cb and ai_error_cb parameters were always passed the
same functions (aiwin_display_response and aiwin_display_error),
making the callback indirection redundant and complicating the call
chain.
Remove ai_callback_data_t, _ai_callback_invoke(), and
_ai_invoke_callback(). Simplify _ai_request_thread and ai_send_prompt
to directly call aiwin_display_error() and aiwin_display_response()
when user_data is a valid ProfAiWin*.