Bind s-~ to ns-prev-frame for Emacs.app

[Matthew Dempsky]

On OS X, it's standard for Cmd-~ to cycle backwards through windows.
Patch below makes Emacs.app follow this convention too.  (Currently
it's not bound to anything.)

--- a/lisp/term/ns-win.el
+++ b/lisp/term/ns-win.el
@@ -208,6 +208,7 @@ The properties returned may include `top', `left',
`height', and `width'."
 (define-key global-map [?\s-,] 'customize)
 (define-key global-map [?\s-'] 'next-multiframe-window)
 (define-key global-map [?\s-`] 'other-frame)
+(define-key global-map [?\s-~] 'ns-prev-frame)
 (define-key global-map [?\s--] 'center-line)
 (define-key global-map [?\s-:] 'ispell)
 (define-key global-map [?\s-\;] 'ispell-next)