The specific BaseItem sub-class that gets instantiated from a
TrackerSparqlCursor used to be auto-detected based on the
nao:identifier. Each sub-class had a matching Source, and a Tracker
miner that set an nao:identifier with an unique prefix that identified
the sub-class.

However, it is not going to be possible to differentiate between local
BaseItems already present on the system and those that are on a device
attached to it because both are going to be indexed by Tracker's
filesystem miner. Fortunately the cases where one needs to create a
DeviceItem, as opposed to a LocalItem, are very well defined.
Therefore, it's desirable to be able to specify the GType of the
sub-class and override the auto-detection machinery.

#29