iOS: Fixed project for upgrading Xamarin.iOS from 7.0.5 to 7.2.5.5.

- Added bass_missing_defs.c and bass_missing_defs.o with new build
arguments to fix linking engine changes for Xamarin.iOS
- Fixed MPfmNavigationController delegates after new Xamarin registrar.
- Updated all BASS librairies for iOS (2 Jul '14 build)
Fixed several bugs in DecodingService.

- DecodingService: Fixed skipping to next song, added possibility to
flush and set a new decode position, fixed bugs in thread
synchronization with worker thread in multiple situations, added proper
disposing of decoding channels.
- Updated DecodingServiceTest.