HasDropDown: emit events just like DisplayContainer, fixes #301.

Emits:

- delite-display-load
- delite-before-show
- delite-after-show
- delite-before-hide
- delite-after-hide
delite/popup: remove code to call popup's onOpen() method, refs #301
Rename HasDropDown#focusOnOpen to _focusOnOpen.
It's a private method.
 delite/popup: remove code to call popup's onClose() method, refs #301
HasDropDown: update reference doc, removing mention of handleKey()
KeyNav: add focusChildren=false mode for ComboBox
Sets d-active-descendant class on navigated descendant.

Refs ibm-js/deliteful#331.
KeyNav: rename focus --> navigate, child --> descendant.

Changed method names:

focusChildren --> focusDescendants
focusedChild --> navigatedDescendant
childSelector  --> descendantSelector
focusFirstChild --> navigateToFirst
focusLastChild --> navigateToLast
focusChild --> navigateTo
onKeyboardSearch --> _keyboardSearchHandler
KeyNav: remove keyHandlers hash; figure out method name automatically.

Changed the handler method names too:

- onLeftArrow --> previousArrowKeyHandler
- nextArrowKeyHandler --> nextArrowKeyHandler
- downArrowKeyHandler --> downArrowKeyHandler
- upArrowKeyHandler --> upArrowKeyHandler

Fixes #304.
document events dispatched by DisplayContainer
update version to 0.4.0-alpha.2