Solved the Ctrl-F, Alt-Tab, Alt-Tab hangnail.
Rewrote and simplified onDeactivateEvent and onActivateEvent. All tests pass.
OnDeactivateEvent remembers the name of the widget with focus.
OnActivateEvent puts focus in the tree if the previous widget was the tree or a headline.
Otherwise, focus goes to the body pane.

Leo build: 20140829164717