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 give us margin for API expansion without breaking ABI.
The ABI restriction will only come in place once we do the first
official release (i.e. 0.x.0).