VideoCaptureDemo: Select 'realmax' as framerate by default.

So device can auto-select a suitable framerate.

Successfully tested with GStreamer engine (camerabin1) under Linux
with:

- Basler A602f and A312fc firewire IIDC cameras. (select 54 fps)
- Unibrain Fire-i with dc1394 video capture engine instead of GStreamer.
- Sony Playstation-3 Eye USB camera. (selects 60 fps)
- Logitech USB WebCam C170 (selects 30 fps)
PsychTweak: Fix comments so 'help PsychTweak' works on Octave.
Screen/GStreamer: Make user-defined videosource spec strings work with DV cameras.

-> Need to avoid auto-creation of ghost-pads.
-> Add special case handling for videocapture with camerabin1 if the
   exact NTSC-DV framreate of 30000/1001 fps is requested, otherwise
   start of capture on NTSC-DV cameras may fail with camerabin1 on
   Linux or OSX.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
-> This one works with IIDC firewire cams, USB cams and our DV-PAL cam :)
New demo: VideoDVCamCaptureDemo.m - Show video capture/recording from DV cams.

This demo demonstrates video capture and video recording from consumer
class DV cameras via the GStreamer engine.

It allows both built-in capture via usual deviceIndex, but also allows
capturing via a user defined videosource string in gst-launch style syntax.

This has been extensively tested under Linux with both camerabin1 and
camerabin2, regular and gst-laumch style use. Testing was successfull
on a PAL-DV camcorder, specifically the "Sony DCR-TRV900E PAL"

Test on MS-Windows with same DV camera pending. No testing with NTSC
equipment possible.