Initialize GES

Without this, PiTiVi may segfault during assets initialization.
Updated Brazilian Portuguese translation
medialibrary: Make rectangle selection work
medialibrary: Handle Iconview selections when pressing Ctrl/Shift
Updated Slovenian translation
Add a contextual help button in the render dialog to explain container formats
Allow entering a frame number into the time widget

The downside to this is that we don't seek live while typing anymore:
the frame number being typed would always get overridden by timecodes.
Users have to activate the text widget (press Enter) to seek with it.

On the other hand, this is compensated by the fact that we also seek
when the user defocuses the text widget.

Fixes bug #684236
Updated Polish translation
project: Make the "Restore from backup" dialog modal to the main window
Updated Slovenian translation
Improve debugging for drag & drop in media/effects library and clip properties
clipproperties: Prevent drag and drop "ghosts" from the effects library

Ensure that dragging and dropping an effect triggers the drag_end signal;
using the "DROP" (or "ALL") flag for default DragDest allows us to correctly
signal the end of the drag and drop without calling drag_finish() ourselves.

Fixes bug #695894
Updated Hungarian translation
Updated Czech translation
Get rid of deprecation warning using latest pygobject
thumbnailer: Don't set a property we don't implement

pointer-events is a virtual property in latest GooCanvas. For this reason, we can't set this
property unless we implement it.
Update git build script to Gnome 3.8
pitivi-git-environment: Update release tags before checking them out
Updated Spanish translation
Port to the new GESContainer API