Get rid of _onFocus(), _onBlur(), onFocus(), and onBlur().
Instead, emit delite-activated and delite-deactivated events on
widgets as they become or stop being active.

Widgets that want to monitor when they become/stop being active should
listen to those events.  Note though that often listening to focusin/focusout
is better.  The delite-activated and delite-deactivated events are mainly for popups.

Other changes:

* renamed focus.js to activationTracker.js
* stopped setting "focused" property on active widgets, and removed "focused" from default list of properties CssState tracks
* fixed wording for activationTracker.js (previously focus.js) API comments and reference doc
* removed focus() method from activationTracker.js; it wasn't being used.

Fixes #214.