Fix #818.

Fix #818.

Use unsmob_engraver ()/unsmob_performer () instead of
explicit casting in translator-group.cc when filtering
translators added via \with { }

* lily/include/performer.hh, lily/performer.hh

  add unsmob_performer ()

* lily/translator-group.cc (filter_engravers/filter_performers):

  use unsmob_* () instead of dynamic_cast when checking
  translator is of correct class to remove