Set our Gst dependency to 1.4.0

We actually depend on 1.4.0, not 1.4.3

https://bugzilla.gnome.org/show_bug.cgi?id=738378
check: remove ClutterGst

https://bugzilla.gnome.org/show_bug.cgi?id=738078
pitivi: Remove our hard dependency to GdkX11

Allowing us to run without X11, as on Mac OS X

https://bugzilla.gnome.org/show_bug.cgi?id=738767
Updated Slovak translation
clipmediaprops: Handle potentially broken framerates such as 1000 fps

Fixes bug #721888
check: clarify Libav as GStreamer Libav
i18n: clarify Libav as GStreamer Libav

This change updates the "formal names" in i18n files
to match the message change in check.py.
Show an error dialog when encountering Unicode decoding errors/broken locales

Fixes bug #729523
viewer: Try to import GdkX11 as it will be needed to embed window on X11

Fixing:
  AttributeError: 'gtk.gdk.X11Window' object has no attribute 'get_xid'
check: Bump the GNonLin and GES dependencies to 1.4.0
ui: Make media/effects library search entries placeholder text translatable
viewer: Do not forget to set ViewerWidget sink when setting our pipeline

Making the expose call working when we need to redraw the viewer.

+ Rename Pipeline._glimage_sink to Pipeline.videosink and making it
  public and simpler to read.

Conflicts:
	pitivi/mediafilespreviewer.py
check: Do no require GNonLin if GES version >= 1.5.0

As it will be directly integrated in GES itself
check: Fix variable naming
viewer: Do not call the expose function when we set the aspect ratio

During caps renegotiation resulting from the change of the video
height/width, aspect ratio changes, there is a race in glimagesink where
we end up with the viewer displaying broken frames calling the expose()
method of the sink at the same time.

Avoid calling the videosink.expose() while we change the resolution
in the pipeline.

https://bugzilla.gnome.org/show_bug.cgi?id=739145
Updated Polish translation