Implemented sentinel events
Removed unneeded clock.tick
Transitioning to STOPPED or COMPLETE now clears sentinels
Merge branch 'new-media-api-sentinels' of https://github.com/fmtvp/tal into new-media-api-sentinels
Should be seeked sentinel now also used in the PAUSED state
Merge branch 'new-media-api-sentinels' of https://github.com/fmtvp/tal into new-media-api-sentinels