New preferred source(s) option

[ccatterina]

Now the options for preferred source are:

• Choose automatically
• Always

We could add a new option to set one or more preferred sources that take precedence over any other players, but if not present then fallback to anything else playing.

In plasma-workspace they're working to add a similar feature to the Media Player widget. That should simplify a lot the development of this in plasmusic, because it adds the preferred player concept to the Mpris model.