User and media playback source information

• SRG Analytics now provides an SRG Identity companion framework. If your application uses SRG Identity to connect to a user account, you should instantiate your tracker with -[SRGAnalyticsTracker startWithConfiguration:identityService:]. This ensures that an account identifier is then automatically added to measurements when a user is logged in.
• SRGPlaybackSettings has an additional sourceUid property, which can be used to relate media playback events with a source, i.e. where the media which is played initially came from. This can for example be useful if the media being played was delivered by some system (e.g. a recommendation engine), provided it identifies the source (e.g. a recommendation list identifier) as a unique identifier.