Shuffle playback support:
- Store list of not-yet-played-tracks and choose from these randomly
- Generate this list upon turning on shuffle (excluding current playing track)
- Double clicking to play a specific track resets this list (excluding current playing track)
- Repeat will regenerate this list once it has run out (excluding previous played track to prevent a double play)
- Known bug: titlebar buttons and media keys (play/next/prev) do not respect shuffle behaviour