delete translation when sentence is deleted.
set default of sentences.imported to false.
only execute callback on axiom when appropriate

Where appropriate means whether the
axiom was defined inside of an <Axioms> or <ImpAxioms>
element inside DGXML. Every other case results
in the callback not being executed.
Merge branch '676-translate_sentences' into develop
Merge branch 'develop' into develop.ontohub.org