This could happen, e.g. with this message order:

  >>> updateTranslationUnitsForEditor()
  add job<1>
  run job<1>
  >>> updateVisibleTranslationUnits(Utf8String(), {})
  >>> updateVisibleTranslationUnits(path, {path})
  add job<2>
  finish job<1>
  run job<2> -- Ops, nothing is changed but job<2> is started

This led to an outdated translation unit (e.g. wrong highlighting).

Now JobQueue checks for duplicates in the queue and checks all the
currently running jobs.

Change-Id: I05843fddcbd21ce0489681c283227c0027ded428
Reviewed-by: David Schulz <david.schulz@qt.io>