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'
GNL is dead, do not check if it is present!
undo: Undo redo logic for assets is now implemented in project.
undo: Disconnect from the signals correctly.
mainwindow: improve signal disconnecting.
WIP for scenarios
scenarios: add the type of the added asset
scenario: serialize action even if running
scenarios: serialize asset removed
scenarios: adding asset requires an id, not an uri
undo: disconnect correct handlers when layer is removed.
undo: No need for class attribute actions.
undo: Add LayerAdded and LayerRemoved actions.