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>