Screen/GStreamer: Allow selection of audio-sink during movie playback.

Introduce a new optional parameter 'movieOptions' to Screen('OpenMovie',...);
This allows to pass in an option string with key=value pairs of additional
playback options.

The only currently supported option is 'AudioSink=<sinkspec>', where
<sinkspec> must be a gst-launch style description of an audio-sink
plugin and parameter, e.g., 'AudioSink=pulseaudiosink device=Foo'
would use the pulseaudiosink plugin for audio output during movie
playback, and ask for output to audio device 'Foo'. This allows
in theory to control audio output routing if a system has more than
one soundcard or more than one output per soundcard.

Screen verbosity level 4 now also reports which audio device was
actually used during playback, to help debug audio routing problems,
e.g., because audio is routed to the HDMI output of the graphics card,
instead of the speakers, and one just wonders why there isn't any sound?

In practice, at least on KDE + PulseAudio on Linux i wasn't able to
override the selected order of preferences for audio outputs on my
MacbookPro. If i set the preferred audio output to DisplayPort/HDMI in
the KDE sound setup control panel, but asked PTB to play out over regular
speakers, the system ignored my wishes. Audio routing was properly set
at start of playback to the PTB spec, but immediately after start of
playback, the PulseAudio server reverted the audio sink to HDMI, ie.
overriding my settings. Soo, this new feature may be of limited utility
at least on some setups :(

-> Screen.mex for 64-Bit Octave on Linux rebuilt and tested -
   with limited success.

-> Another rather unfunny insight is that delivery of GStreamer bus
   messages doesn't work if Octave-3.8.1 is used with its GUI, at
   least for the GStreamer 0.10.36 installation of Ubuntu 14.04 LTS.
   It works with Octave 3.8.1 in command line mode, and with Matlab
   R2012a in GUI mode though. So something fishy is going on here with
   Octave GUI.

   Without proper bus message delivery, error handling is impaired,
   e.g., doesn't report why opening a movie file failed. Also looped
   video playback at default settings doesnt'w work, because the
   required EOS messages aren't received from the bus. Only method 64
   works, as it doesn't need the bus.

-> Another bad day in the life of our hero :(