Highlight all instances of an OTU in context, in the tree viewer

This provides a "stepwise" UI for reviewing an OTU in context, as an aid to mapping it to OT taxonomy. (The same tools will be used to choose between conflicting non-sibling instances within a single tree.) I'm also showing tentative and original OTU labels in the tree view, with distinctive style for each label type, and a full explanation in the node-menu popup.