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