Only current song

Added an option so the app only shows the current playing song in the notification center (dismissing the rest) so it doesn't get full of previously played songs.
It also dismisses the notification when pausing if the Play/Pause Notification is enabled.
When the user fires the notification with the shortcut, it makes sure there is only one notification in the notification center and in case there is no song playing it shows the notification but it doesn't keep it in the notification center