Bitcoin-Qt: ensure createTrayIconMenu() is always called on Mac

- this should prevent GUI issues on Mac that were observed before (disappearing
  GUI - see #1522)
- the patch ensures, that createTrayIconMenu() is always called on Mac to
  process and use our MacDockIconHandler
Merge pull request #2386 from Diapolo/Mac_GUI

Bitcoin-Qt: ensure createTrayIconMenu() is always called on Mac