Unify shutdown handling under NS, via ns-power-off key event (where appropriate)...

Unify shutdown handling under NS, via ns-power-off key event (where appropriate) and save-buffers-kill-emacs.

remove  NS-specific preferences window that used to show NS-specific preferences.
Integrate preferences in Lisp accessible customization system where possible.
Remove ns-expand-space, because it can be set only during fontloading (i.e. not at runtime),
and because of redundancy with line-spacing frame parameter.

Remove loading of most and saving of all NS resources (from org.gnu.Emacs.plist).

nsterm.m (ns_shutdown_properly, -terminate): remove global state variable as it was never reset
(ns_term_init): remove initialization of Lisp-settable defaults and ns_expand_space
(-setPanelFromDefaultValues): remove ns_expand_space.
(-showPreferencesWindow): send new KEY_NS_SHOW_PREFS key.

nsfont.m (nsfont_open): remove ns_expand_space, assume -0.5 i.e. no additional spacing, similar to Carbon port.

nsterm.h: define KEY_NS_SHOW_PREFS key.
nsfns.m (ns-popup-prefs-panel): remove.

term/ns-win.el (ns-expand-space): remove.
(ns-show-prefs event): run `customize'.
(ns-power-off): run `save-buffers-kill-emacs', but ask user whether to save files (as is standard)
(clipboard-yank, clipboard-kill-ring-save, clipboard-kill-region)
(menu-bar-enable-clipboard): do not undefine these.
(ns-save-preferences,ns-save-options,ns-show-preferences-help): remove.