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/libav/default.nix

@@ -3,10 +3,13 @@
 , withSystemLibav ? true, libav ? null
 }:
 
+# Note that since gst-libav-1.6, libav is actually ffmpeg. See
+# http://gstreamer.freedesktop.org/releases/1.6/ for more info.
+
 assert withSystemLibav -> libav != null;
 
 stdenv.mkDerivation rec {
-  name = "gst-libav-1.4.5";
+  name = "gst-libav-1.6.1";
 
   meta = {
     homepage = "http://gstreamer.freedesktop.org";
@@ -17,7 +20,7 @@ stdenv.mkDerivation rec {
 
   src = fetchurl {
     url = "${meta.homepage}/src/gst-libav/${name}.tar.xz";
-    sha256 = "1g7vg9amh3cc3nmc415h6g2rqxqi4wgwqi08hxfbpwq48ri64p30";
+    sha256 = "1a9pc7zp5rg0cvpx8gqkr21w73i6p9xa505a34day9f8p3lfim94";
   };
 
   configureFlags = stdenv.lib.optionalString withSystemLibav