Introduce ns-auto-hide-menu-bar to hide menubar for Emacs frames.

Introduce ns-auto-hide-menu-bar to hide menubar for Emacs frames.
Code by Anders Lindgren.

* nsterm.m (ns_menu_bar_is_hidden): New variable.
(ns_constrain_all_frames, ns_menu_bar_should_be_hidden)
(ns_update_auto_hide_menu_bar): New functions.
(ns_update_begin): Call ns_update_auto_hide_menu_bar.
(applicationDidBecomeActive): Call ns_update_auto_hide_menu_bar and
ns_constrain_all_frames.
(constrainFrameRect): Return at once if ns_menu_bar_should_be_hidden.
(syms_of_nsterm): DEFVAR ns-auto-hide-menu-bar, init to Qnil.