Support for observing and setting the play position of an audio clip

[git-moss]

Observation currently only works for MIDI clips.

Progress of the playback phase for each of the currently playing Clips could be indicated.

Setting the play position would allow to implement scrolling forwards/backwards like a tape machine (important for DJing)

[git-moss]

Playback progress for all clips (e.g. between 0..1) would be a good start.