Implement a fully functional MPD status listener that works without libuv

[promi]

There are currently two status listeners:

1. src/mpd/status-listener-uvw.cpp

   Implemented with uvw / libuv and only calls the status callback when there is an actual change on the MPD server. It uses modern event notification mechanism (epoll (), etc.) through libuv.

2. src/mpd/status-listener.cpp

   Implemented using the normal POSIX socket functions, but is not complete. Currently it calls the status callback with a dummy Idle::Song flag set every 2 seconds. It should actually call MPDs idle method (like the uvw one does) and preferably use select (). At the beginning a simple loop whould be fine as well.