search: add a dialog to configure Tracker search locations

Ideally applications could configure Tracker independent from each
other, but we don't have such a feature currently.
In the meanwhile, bring up a settings dialog that can allow
configuration of the locations indexed by the Tracker files miner, as
suggested by the current design.

https://live.gnome.org/Design/SystemSettings/Search

https://bugzilla.gnome.org/show_bug.cgi?id=687490