Prefer attachedCallback() to startup().
Also, make createdCallback() and attachedCallback() automatically chain.
Removes chaining of startup() though, since subclasses generally shouldn't
extend it anyway.

Fixes #287, refs #283, refs #257.
Remove code from emit() to squelch events emitted before startup() call.
Fixes #288.
Remove _onKeyDown() extension point from DialogUnderlay.
User code can just call DialogUnderlay#on("keydown", ...), although in the case of Dialog
it seems easier to just listen for keydown events on <body> or <html> instead.

Also remove dcl.before() advice from destroy() method, since that method
is automatically chained.

Refs #281.
Remove underscore from names of protected properties:

* CustomElement: _created, _attached, _parseAttr --> parseAttribute, _parseFunctionAttr --> parseFunctionAttribute
* HasDropDown: _buttonNode, _arrowWrapperNode, _popupStateNode, _aroundNode
* KeyNav: _onLeftArrow, _onRightArrow, _onUpArrow, _onDownArrow, _getNext
* Widget: _started
Use eventNameHandler or _eventNameHandler for event handlers.
Fixes #281.

Note that _onFocus/_onBlur/onFocus/onBlur is tracked separately as #214.
remove vestigial code from when scroll bar was on popup wrapper, refs #266.