VideoDVCamCaptureDemo: Update, clarify for GStreamer-1.x backend.

Drop dead code, update help text. Add setup instructions for the
video4linux2 loopback device, which seems to be a more robust way
of driving DV cameras on Linux + GStreamer 1.x.

This had some light testing on Ubuntu 14.04-LTS, but no testing
on Windows or OSX, because for OSX i don't have any useable testing
setup, and on MS-Windows the current GStreamer-1.4.0 runtime doesn't
provide any useable video capture plugins, so video capture testing
is impossible, except for the videotestsrc to simulate a video source.