[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.
Merge pull request #5300

a7f2941 [Qt, OSX] fix usage of osx 10.8+ user notification center (Jonas Schnelli)