We should be consistent about what the name of a Tracker domain actually
is. In the domain rule we specify Domain=org.freedesktop (for the
default rule) or Domain=org.example.App (for a custom domain). But
internally tracker_domain_ontology_get_domain() would return
org.freedesktop.Tracker1 or org.example.App.Tracker1, i.e. the domain
name now has '.Tracker1' appended.

This does make sense in most cases, but it means there's no way to get
the actual name of the domain from a TrackerDomainOntology object. This
commit changes the existing function to not append '.Tracker1' in the
value it returns, which means that function can now be used to get the
base name of the domain. The assumption is that callers are normally
appending stuff to this base name anyway so it's not much extra effect
to also append the '.Tracker1' component if needed.

libtracker-common is internal to Tracker so this doesn't constitute a
public API break.