gstreamer: 1.4.x -> 1.6.x (all modules)

(And while at it, gst-vaapi 0.6.0 -> 0.6.1.)

* gst-editing-services grew additional build time dependencies, flex and
  perl.

* gst-libav switched from libav to ffmpeg as "libav" provider, see
  http://gstreamer.freedesktop.org/releases/1.6/.
  Without using ffmpeg, one may hit issues such as this (which I
  initially did):

  (gst-plugin-scanner:19751): GStreamer-WARNING **: Failed to load plugin '/nix/store/0wgpq2yx9wrkp2mh4rn1c7zbiq2bqa2l-gst-libav-1.6.1/lib/gstreamer-1.0/libgstlibav.so':
  /nix/store/0wgpq2yx9wrkp2mh4rn1c7zbiq2bqa2l-gst-libav-1.6.1/lib/gstreamer-1.0/libgstlibav.so: undefined symbol: av_frame_get_sample_rate

pkgs/development/libraries/gstreamer/default.nix

@@ -15,6 +15,8 @@ rec {
 
   gnonlin = callPackage ./gnonlin { inherit gst-plugins-base; };
 
+  # TODO: gnonlin is deprecated in gst-editing-services, better switch to nle
+  # (Non Linear Engine).
   gst-editing-services = callPackage ./ges { inherit gnonlin; };
 
   gst-vaapi = callPackage ./vaapi { inherit gst-plugins-base gstreamer gst-plugins-bad; };