* Start implementing playlist
Done so far:
* Creation
* Changing playback mode
* Changing repeat mode
* Add skipping for linear playlists with all repeat modes
* Complete implementation for playlist/next-song
* Implement all the playlist skipping functionality
* Add functions to enqueue songs
* Remove start-idx parameter when constructing playlists
* Use namespaced keywords only when modifying external data
E.g. songs in the queue
