separated the DefaultBottomBarViewModel into 2 view models (DefaultBottomBarViewModel and PlaybackBottomBarViewModel) as it had a lot of conditional logic inside it and its views and also extracted the CollectionViewModelBase class inherited by both DetailsViewModelBase and SearchViewModel
moved the add item/items to playlist logic from the playback view model to the model loade extensions class and used it from both playback view model and default bottom bar view model
Merge pull request #53 from cristi-badila/cleanupPlaybackViewModel

Cleanup playback view model