add imported flag to sentences

This flag shall respresent if this
sentence originated in a <ImpAxioms>
element.
set default scope of sentences

Set the default scoped to imported: false
to ensure past behavior. In order to get
all sentences, the all_sentences method
was defined.
add method to determine if logic translations exist

Logic translations have to be present if
at least one of the imported ontologies is not
of the same logic as the base ontology.
add view support for logic-translations.
add import support for imported sentences

This includes a new hets switch and
the parse/import of element <ImpAxioms>.
allow self/all switching for logic translations

Allow switching between self-defined sentences and
all sentences for logic translated sentences.
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.