Use observers to report stop events in MI.

	Use observers to report stop events in MI.
        * mi/mi-interp.c (mi_on_normal_stop): New.
        (mi_interpreter_init): Register mi_on_normal_stop.
        (mi_interpreter_exec_continuation): Remove.
        (mi_cmd_interpreter_exec): Don't register the above.
        * mi/mi-main.c (captured_mi_execute_command): Don't care
        about sync_execution.
        (mi_execute_async_cli_command): Don't install continuation.  Don't
        print *stopped.
        (mi_exec_async_cli_cmd_continuation): Remove.