mu4e: add support for macos notifications

The current order in this patch is

1) macos notification
2) built-in dbus notification
3) fallback text notification

because macOS emacs can actually have dbus enabled (for emacs daemon
mode) but that doesn't support notifications. This is a built-in way to
send a simple notification for macs, too. Yay!

mu4e/mu4e-notification.el

@@ -67,6 +67,9 @@ support."
                             (if (= delta-unread 1) "" "s")
                             (plist-get fav :name))))
     (cond
+     ((fboundp 'do-applescript)
+      (do-applescript
+       (format "display notification %S with title %S" body title)))
      ((fboundp 'notifications-notify)
       ;; notifications available
       (setq mu4e--notification-id