PlaylistController refactorings

• PlaylistController is now a singleton.
• Removed PlaylistController instance from app delegate.
• Eliminated playlist updates via notifications. Subscribers must now use KVO on playlist property of singleton.
• Eliminated loading state enum. Nothing used this and it wasn't fully implemented.
• Lockscreen media is now managed in a separate class.

Signed-off-by: Jake Bromberg <jake.bromberg@gmail.com>
id -> instancetype

Signed-off-by: Jake Bromberg <jake.bromberg@gmail.com>
modified files

Signed-off-by: Jake Bromberg <jake.bromberg@gmail.com>