Add comments for forced L2R directions of menu bar and tool bar.

Add comments for forced L2R directions of menu bar and tool bar.
GCPRO Lisp string inside bidi.c.
Force L2R direction in buffer menu buffer.

 src/xdisp.c (tool_bar_lines_needed, redisplay_tool_bar)
 (display_menu_bar): Force left-to-right direction.  Add a FIXME
 comment for making that be controlled by a user option.
 src/bidi.c (bidi_move_to_visually_next): GCPRO the Lisp string we
 are iterating.
 lisp/buff-menu.el (Buffer-menu-mode, list-buffers-noselect): Force
 left-to-right paragraph direction.