private Column<DocumentType, DocumentType> getTickBoxColumn() {
        return new Column<DocumentType, DocumentType>(new DocumentTypeCell(this)) {
            @Override
            public DocumentType getValue(final DocumentType documentType) {
                return documentType;
            }
        };
    }
@return A set of types to include, or empty if the type filter is not active
stroom-core-client/src/main/java/stroom/explorer/client/presenter/TypeFilterPresenter.java