Implement MI notification for new threads.

	Implement MI notification for new threads.
        * doc/observer.texi (new_thread): Document.
        * observer.sh: Forward declare struct thread_info.
        * thread.c (add_thread): Notify observer.

        * interps.h (interp_init_ftype): New parameter
        top_level.
        (interp_set): Likewise.
        (top_level_interpreter_data): Declare.
        * interps.c (interp_set): New parameter top_level.
        Pass it to interpreter's init function.  Remember
        top level interpreter.
        (interpreter_exec_cmd): Adjust.
        (top_level_interpreter_data): New.
        * main.c (captured_main): Pass 1 for top_level
        parameter of interp_set.
	* cli/cli-interp.c (cli_interpreter_init): New
        parameter top_level.
        * tui/tui-interp.c (tui_init): New parameter top_level.

        * mi/mi-interp.c (mi_new_thread): New.
        (mi_interpreter_init): If top level, register
        observer for new threads.

        * Makefile.in (mi-interp.o, thread.o): Update dependencies.