Screen: Allow setup of videosource bin via 'SetVideoCaptureParameter'.

GStreamer engine can use a gst-launch line bin description for a
videosource if deviceIndex -9 is used. Use a special video capture
parameter setting to define the launch line instead of abusing the
'moviename' in 'OpenVideoCapture'.

-> Allows concurrent use of video recording and videosource bins.
-> Screen for 64-Bit Octave on Linux rebuilt.