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
Add layers on project loading
Make the ruler update only on zoom stopped
Make the ruler update only on scroll stopped
Thumbnailing:
Use JPEG compression for the thumbnails.
Create all the thumbnails in advance even if they are not visible at the
moment.
Scale the thumbnail in the pipeline and not manually.
Respect the in-point of the element when drawing thumbnails. Draw only visible thumbnails.
Implemented wishlist (prioritize the generation of thumbnails that the user can see at the moment)
Fix performance problem
Only commit once
Put the zoombox in the right place
Subclass TimelineElement in ClipElement and TransitionElement
Fix title clips handling.
Make splitting work
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
Remove weird animations when adjusting clip positions.