398 delete ontology button

This branch adds a delete button to the ontology view (#398).

 * An ontology can only be deleted if it is not imported by another ontology (secured in model and controller).
 * When deleting an ontology, its defining file gets deleted from the repository as well.
 * If a distributed ontology is deleted, all its children are deleted.
 * If a child ontology is deleted, the parent distributed one will be deleted, too.

I recently merged staging into this branch to make the automatic merge possible.