Add tracker_sparql_connection_get/set_dbus_connection()

Currently it's only possible to open a TrackerSparqlConnection to an instance of Tracker that is running on the session-wide message bus.

There are use cases for running the Tracker daemons on a private session bus though. In fact it's necessary to do this if you want to set up a custom domain without having to become root and create .service files in /usr/share/dbus-1/services. It would also be useful for the functional-tests to be able to use libtracker-sparql instead of having to talk directly to Tracker's D-Bus API.