Option to hide "abstract" classes in model design UI

[azaroth42]

There are many classes and properties in CIDOC-CRM that are never instantiated directly, such as E1_Entity or E70_Thing. It would be valuable to limit the available terms to only those that are tagged (somehow, TBD) as in use. This could be a visual hint, or a flag that hides them completely.

This would make it easier for both novices (they only see things they should use) and faster for experienced modelers (they can more quickly see the ones they do use)