libtracker-common: Make TrackerDomainOntology totally not an object

Hopefully the last chapter in the "make libtracker-common static" saga... Turn this object into a refcounted struct, as we still may end up with multiple copies of libtracker-common in libtracker-miner, through libtracker-sparql.so and directly as it also uses libtracker-common. This resulted in assertions when trying to register the same type name twice.