TODO-tagsets.txt

maybe "ont tag ont-entry [-]tags..."
convenient ontology editing, something like "add cast_member name detail-tags"
TagsOntology: where do metadata live in a split ontology? ideally the subontology - move to a tag_metadata method to keep the context?
ontologies: subtags should have their tag names prefixed/unprefixed? maybe better to just resolve them locally?
TagFile: should subclass MappingTagSets
TagsOntology: reverse prefix mappings
TagsOntology: accept directory, process files within, deduce prefix from filename, ONTOLOGY.sqlite for the default
TagSet.subtags: subclass PrefixedMappingProxy also
TagSetMapping: base class for TagFile and Entity providing singleton TagSets with modification tracking and autosave
BaseTagSets: rename TagSetClass throughout to new_TagSet for clarity
TagsOntology: reverse prefix mappings