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
Correct some verb tenses in the French translation

"Met en pause" looked particularly odd in the rendering progress dialog
Added Serbian translation
Updated Czech translation
Updated Czech translation
Updated Czech translation
viewer: fix alignment
viewer: Small code and comment cleanup
medialibrary: Fix the import progressbar to show translated text properly

Fixes bug #709537