stbt-tv: Stream live video through network

Launching 'stbt tv' with '-s' command line argument streams video
through a TCP socket. Used in conjunction with 'stbt control',
it makes remote configuration of the set-top-boxes under test possible.

TCP port is read from the new 'global.live_stream_port' parameter of
the stb-tester config file, or the hard-coded value 4953 is used if it
isn't specified. 4953 is the default value used by 'tcpserversink'.

Video bandwidth is read from 'global.live_stream_bandwidth' parameter
in bps, defaults to 1500000 (1.5 Mbps). This value is a rule of thumb
compromise of video quality and network load; users can fine-tune the
bitrate with tools like tcpperf.

The receiving side can watch the video stream with
* vlc:
    `vlc tcp://<host>:<port>`
* gst-launch:
    `gst-launch tcpclientsrc host=<host> port=<port> ! \
        mpegtsdemux ! decodebin2 ! autovideosink`