[Tracker] domain-ontologies, continued

[Carlos Garnacho <carlosg gnome org>]

Hey hey,

During the past week I've been continuing the stuff that Philip
started on wip/domain-ontologies. I pushed my current progress on
wip/carlosg/domain-ontologies:

https://git.gnome.org//browse/tracker/log/?h=wip/carlosg/domain-ontologies

So far it's shaping up nicely, private databases are possible there
through the public tracker_sparql_connection_local_new(_async) calls,
xsd/dc/rdfs/nrl/nao are the are loaded from GResource and are the base
ontology, the local connection will run a dedicated thread for
updates, in a very similar fashion to tracker-store itself. My topmost
items in the todo now are:

- TrackerDataManager (and many other subsystems) is still a singleton,
which doesn't play nicely if you can now create multiple connections
that require it differently. I'm halfway into having it be an
object/struct so each connection can get its own instance.

- Lots of documentation need to be written around this: how to create
new ontologies, data migration concerns, dos and don'ts, ...

- Even if some apps could take advantage of private databases, for
some scenarios we do need to make it possible running a standalone set
of tracker dbus services for private use. I'm still unclear on how to
make it most transparent to apps, probably through libtracker-control
API.

There's of course more items for the longer term, but all tests pass
with no functional changes, so seems good enough for an update :).

And btw, I still think it makes sense to tag tracker-next as 2.0, and
use the opportunity to switch to semver, I do hope it plays out and
reduces some maintenance burden in maintaining multiple versions.

Cheers,
  Carlos