gapless: Make play/next/previous gapless

This involves using EOS events to trigger all track changes, or no longer using playbin. Since we don't want to ditch playbin just now it basically works out to having to trigger an EOS in audio, and then just expand our about-to-finish logic to handle play/next/previous cases. Additionally we will need to flush any queues by doing a flushing seek without start or end set.

Once this is solved we will have true gapless with respect to streaming.