Move player scripts from OS into Snakeoil API

Currently players are configured and started from a shell. This was legacy code from the original Snakeoil code and is not as scalable.

Now that we are using Qt, we should upgrade the mechanism and move the shell scripts into the API, this allows us to do code reuse (e.g. all MPD can be placed under the same super class, the same JRMC class can support different releases and so on).