Support for showing one or more highlighted nodes.

This is done in the tree-view popup, whenever we want to highlight one
or more nodes or OTUs in their phylogenetic context. This might be to
clarify the placement of an OTU in all trees, or to choose among
competing OTU mappings within a single tree. Needs work!
Stepwise node display and popup management.

This also addes tentative (un-mapped) node names in the tree view, for
a more clear presentation.
Merge branch 'master' into highlight-nodes-in-tree-viewer
Richer labeling of tree nodes and node-menu popups.
Merge pull request #269 from OpenTreeOfLife/highlight-nodes-in-tree-viewer

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