renderer: Support m3u playlists

Some internet radios use that as stream redirector and it's the preferred
playlist format of UPnP.
renderer: Check instance id of prev/next

Signed-off-by: Jens Georg <mail@jensge.org>
renderer-gst: Stop() and Pause() never transition

Cf. Figure 1, AVTransport:2 DCP

Signed-off-by: Jens Georg <mail@jensge.org>
renderer-gst: Only try to parse non-empty meta-data

Signed-off-by: Jens Georg <mail@jensge.org>