Port to the new little API changes in GES

GES.TimelineLayer has been renamed GES.Layer
The rate property of GES.TimelineLayer.add_asset has been removed
Begins the integration of the new ClutterTimeline
Make the ruler update only on zoom stopped
Make the ruler update only on scroll stopped
Various performance improvements to the timeline clip thumbnailer

- Create thumbnails in the background (even when not visible at the moment)
- Draw only visible thumbnails, respect the clip's in-point
- Prioritize visible thumbnails over background thumbnail generation
- Use JPEG compression for thumbnails
- Scale the thumbnail in the pipeline instead of doing it manually
Improve clip thumbnailing performance when scrolling/zooming the timeline
Only commit clip thumbnails once they're done processing
Subclass TimelineElement in ClipElement and TransitionElement
Fix the handling of title clips in the timeline
Make actions sensitive on selection changed
Re-enables snapping. Also disconnect from signals when needed.
Update Stage Width according to timeline duration
Remove unused imports, and separate elements and previewers from the timeline code
Code is now clean and consistent, and logically split in different files
Remove obsolete View and Controller classes
Update Makefile.am to take additions into account
Reimplement layer removal in the Clutter timeline
Make the default interval of positionListener shorter
Save resources by only drawing the visible thumbs
Fix the Thumbnails if a clip is used more than once