element.py: Manage scheduled tracking state in Element

This removes the scheduled state of tracking from Sources, as
this is really an element wide thing.

To be consistent with assembly, now this comes with:

  o Element._schedule_tracking()
  o Element._tracking_done()
  o Element.__tracking_scheduled
  o Element.__tracking_done

Updated the TrackQueue() to call Element._tracking_done() similarly
to how we have BuildQueue() call Element._assemble_done().