The API is simplified: we can play a song, restart the currently playing song, load a song (that'll be used only by local storage), play the next track or the previous track.
As a result, I changed all the calls to playSong(false, song) which are now just play(song)
However, saving track's position is broken.
Some are still there because of problems I don't know how to solve, e.g. circular dependency between notifications and player.
Uses the queue controller for the sidebar queue.
Moves loadTrackPosition to the main controller.
