document and test HasDropDown key forwarding, fixes #295
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.
1. delite-activate --> delite-activated, and same for delite-deactivate
2. clarify api comments in focus.js
stop setting "focused" property
rename events emitted from delite/focus itself.

widget-focus --> activated
widget-blur --> deactivated
focus.js --> activationTracker.js
delite-activated --> activation-tracker-activated,
delite-deactivated --> activation-tracker-deactivated,