Add helper functions to set/retrieve the application badge

These helper functions wrapp the necessary objective-c calls to set or
retrieve a text (OS X) or number (iOS) on the application icon, also
known as badge.

Change-Id: I34a82c5abc073b63442e12c49a864e65b5428d83
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>