Ontology editor: remove superfluous option from some context menus

For the sake of simplicity, the option to add a new class or relation
should only be available in the context menu of class and relation
tree's root nodes.
Allow ontology relations to be renamed
Ontology editor: add relation renaming support
Allow ontology classes to be renamed
Ontology editor: add class renaming support
Merge branch 'rename-option-for-ontology-editor'