trackIndex prop for PlayPauseButton

[benwiley4000]

This optional prop will turn a PlayPauseButton into a control for a specific track in the playlist - it will only display as playing if the selected track is playing, and hitting play will switch to the new track.

Also consider whether it makes more sense for this to be a separate component.

This is useful in a playlist view where we want to see a control for activating each track, and an indication of which one is playing.

[benwiley4000]

Short interaction spec

Hitting the button will:

• If track is current:
  • Call onTogglePause (current behavior)
• If track is not current:
  • Call onSelectTrackIndex