Use ng-model when communicating state to/from the
tree in browse mode. This will simplify implementation
of the Locator control, which also uses a tree, but
which should not set navigation state. WTD-593.
Add in-line documentation to TreeNodeController, and update
glossary with some clarifying definition.
Additionally, change name from tree-item to tree-node for
consistency.
Part of ongoing transition of commonUI bundles, WTD-574.
