libtracker-miner: Ensure the decorator nie:DataSource is queried as an IRI
libtracker-common: Add define for the tracker: ontology prefix
tracker-extract: Use a fully expanded name for the decorator nie:DataSource

This is in order to avoid confusions where the TrackerDecorator gets the real
nie:DataSource ID for comparisons in GraphUpdate, but implementations insert
the shorthand, that doesn't end up resolving to the same nie:DataSource.