Same requirement as for GStreamer

Allows moving independently (or not) timelineobjects and trackobjects and
have them synchronized with the offsets taken into account.

Right now only the start and priority properties are synchronized. The duration
and in-point properties will require more thoughts.

And make sure notifications are emitted at the right time

And update all code using it

Add needed setters/getters

This should make GES gobject-introspection compliant now.

The refcount isn't incremented.

And ensure exported symbols are properly documented and have
argument checking.

fires

Added by error when I added the doc system

It wasn't making us gain anything, and confuses the hell out of g-ir-scanner.

Remove dependency on gst-convenience.

API: GESTrackObjectClass::gnlobject_factorytype
API: GESTrackObjectClass::create_element

Most track objects are only specific by the contents of the gnlobject,
therefore move the 'create_element' vmethod which was already present
in some subclasses to the top-level class.

Also make the code more robust

This will be made more generic by allowing any overlay/effect to
be put on any source object.

And detect when we're trying to add contents to a gnlsource which
already has something

This is to ensure people can create their own Layer Transition subclass.

API : GESTimelineTransition is now GESTimelineStandardTransition

This is a new class for all timeline objects that both produce and
consume data.

The existing subclasses of it are now:
* GESTimelineOverlay
* GESTimelineTransition