Screen: Fix video device enumeration via GstDeviceMonitor/DeviceProvider.

We did it wrong. Now it's fixed. Both, enumeration on a per-plugin
class via GstDeviceProvider and catch-all enumeration of everything
via GstDeviceMonitor works, as tested with Linux Video4Linux2 v4l2src
with a iSight under Ubuntu 14.10, GStreamer 1.4.0.

-> Device monitor enumerates everything it finds under classIndex 0,
   starting with inputIndex (=deviceIndex) 1:

   DeviceIndex 0 = First detected video source.
   DeviceIndex 1-n = device monitor enumerated sources.
   DeviceIndex classIndex * 10000 + n = Sources from class/plugin, either
   via GstDeviceProvider or via active probing or via passive addition.

-> Currently only v4l2src for Linux Video4Linux2 supports this stuff,
   all other plugins are either actively probed, or passively added,
   or not detected at all.

-> Also switch from active probing of [h]dv1394src DV sources to passive
   addition of 1 fixed GUID 0 source if plugins are available. Active
   probing didn't work well at all.

-> Add legacy warning when compiling against GStreamer-0.10.x, for safety.

-> Screen.mex for 64-Bit Octave on Linux rebuilt on Ubuntu 14.10 against
   GStreamer-1.4.0, for testing of device monitor/provider based enumeration.
   Works! Must be removed from production release though, where we'll build
   against GStreamer 1.2.4 instead, without builtin device monitor/provider
   support, so Screen() works with older GStreamer < 1.4.0 runtimes that ship
   with distros older than Ubuntu 14.10 or upcoming Debian-8. We'll switch to
   GStreamer-1.4.0 when the NeuroDebian team provides backpors of GStreamer 1.4
   on all relevant distros.