Player: Added support for push streams.

- Commented out the use of push streams, because it is not completely
working yet.
Fixed bugs with peak file generation cancel.

- Added test cases for PeakFileService.
iOS: Fixed peak file generation canceling, fixed AirPlay metadata.

- Fixed peak file generation canceling when skipping songs.
- Also now cancelling peak file generation when the app is put to sleep.
This prevents iOS killing the app in background because of CPU usage.
- Several bug fixes in AirPlay/lock screen metadata by using a new Now
iOS: Several bug fixes in UI.

- Also a few fixes for the app the build after changes on other
platforms.
- MVP: Added playlist refreshing for filling the Playlist view on
desktop platforms.