add http-stream support, URL is hardcoded for now

Hi Andi, this patch needs some follow ups:
- need different set of UI controls for mpd and local playback control (start/stop/volume)
- the stream url is hardcoded to some url, dont know whether mpd provides this info
On the other hand:
maybe better to make the stream URL configurable on the client side:
if mpd also streams to a broadcaster, one wants to check whether the stream is "on air" too.