Display and send metatag information

For the upcoming release 0.3.0 we need support for metatags:

* Display metatag information on the application window
* Send metatag information to an Icecast server
* Check for the presence of mediaplayers and streaming clients

To achieve this we need a loop at runtime that checks for a media player running, gets information on track change when streaming, display metatag information on the GUI, and sends them to an Icecast server if present. This can be done via d-bus for media players that use a standard implementation of MPRIS. To display the cover art we need a reproducible path where to look for image files.

From the loop that watches for track changes we can add an is-alive check for streaming clients (relevant to Airport Express and Bluetooth devices) and thus display the streaming status more precisely.

The implementation should be optional on user settings.