Properly handle no track being active

* If you remove the currently playing song, and it's the last song in
  the playlist, then the playlist seeks to position -1

* Seeking to position -1 sets the "playing song" model's song ID to -1

* When the playing song view sees an ID change to -1, it stops music,
  hides the playback buttons and resets the background image to default

* This commit also fixes buggy behavior when loading an empty playlist
  from LocalStorage