Don't execute queries over the tracker dbus interface.

Pushing a few thousand query results over a dbus connection all at once
is inviting failure, and somewhat slower than the alternative. Don't do
it.

Change-Id: I6114cd4ee4d55c874cb806446c995e458596d906
Done-with: Andrew den Exter <andrew.den.exter@jollamobile.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Fix assertion on construction.

Ensure g_init_type() is called before attempting to use any of the
tracker_sparql api.

Change-Id: I2d71c362bc43fb1f51a6d66c4d26daf3a45e5999
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Verify the type of query results before accessing them.

Tracker will emit ugly warning messages if the wrong accessor is used
to access a value including when the result is null, so always check
the type before extracting a value.

Change-Id: I4df575aa742fd047e7fd349f43339d999305a2c1
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>