add view support for logic-translations

Also allow self/all switching for logic translations.
This should make switching between self-defined sentences and
all sentences for logic translated sentences possible.
add import support for imported sentences

This includes a new hets switch and
the parse/import of element <ImpAxioms>.
use count-method for sentence/symbol count-display

The .size method gets 'confused' when using
default_scope (since it does not apply the scope (correctly)).
do not rely on timestamp when clearing ontology

When importing an ontology we should not
rely on timestamps when clearing the entities/symbols
and sentences. Instead we should delete all of them
(for an ontology) before creating new ones.

We also call destroy on sentences, since
we want the callbacks to be executed.
bump minimal hets version to revision 18553

Since we now use the --full-theories switch, we
need a hets version which supports it.
fix logic translations question

Both parts of the UNION need to
select logic_ids in order to work properly. So
we shall not pass the ontology.id, but the
ontology.logic_id to the query.
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.