debugging output and cosmetics
flush and re-fetch the AudioPrefetch.

All WaveTracks are now played back accurately, newly
created Tracks will not be "in the future".
This partially fixes muse-sequencer/muse#338
However, there will be still some silence instead of audio
when inserting new events!
flush/re-fetch AudioPrefetch upon Event/Part change

makes sure to incorporate the lastest WaveEvent/WavePart
movements in the played back Audio. If a change would change
the current FiFo content, the fifos are flushed and rebuilt.
This addresses and hopefully fixes muse-sequencer/muse#338