tracker: Rewrite tracker GrlSource notification to use TrackerNotifier

This is a new facility in Tracker abstracting usage of the GraphUpdated
dbus signal, so is a perfect fit to use here. The event resources are
rather plainly translated to GrlMedias so they are emitted.

Events are also grouped so the tracker source makes use of the
grl_source_notify_change_list(), which will avoid high signal emission
overhead if there's many changes to notify about.

Closes: #14
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00:04:11