DBus, NP Signals and persistence

- Split up the Spotify NP source into a DBus mpris base source and a Spotify-specific source extending the mpris source.
- Added support for signals for the now playing source. This allows us to send play/pause, next etc. to the primary NP source.
- Added the possibility to control persistence for a WebSocket message. Persistence is by default on, but can be deactivated by setting the 'persist' key to False.
- Added a new contrib script for sending signals to the currently active NP source.