Text sub-system failed to initialize.
Text sub-system is ready; all embedded tracks have been registered.
Text sub-system is initializing for the current media (start of setup).
An individual sidecar text track failed to load.
An individual sidecar text track finished loading successfully.
An individual sidecar text track has started loading.
The active text track has changed.
An individual sidecar text track has been unloaded.
Text sub-system has been torn down and all tracks removed.
Fired after any change in text sub-system (catch-all change notification).