Get rid of _onFocus(), _onBlur(), onFocus(), and onBlur().

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.  Sort of like how dpointer generates synthetic
events.

Technically I guess the events should be called focus-activate and focus-deactivate,
since they come from the delite/focus module, but that seems confusing because the
events aren't about focus/blur per se.  I'd actually like a better name for the delite/focus module,
but not sure what.

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.