[Qt, OSX] fix usage of osx 10.8+ user notification center

Currently Bitcoin-Qts support for OSX User Notification Center is broken. This pull will fix a known issue of non-official-apple-built apps having problems sending user notifications.

Again this is a unloved platform dependent thing but we might continue to support this because we started this in #2615.
Nit: Spelling ``method`` and at the end ``identifier`` :).