Get rid of _onFocus(), _onBlur(), onFocus(), and onBlur().
Instead, delite/focus emits delite-activate and delite-deactivate 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-activate and delite-deactivate events are mainly for popups.

Fixes #214.