use data item for selection in doc to be consistent with what
we are doing in deliteful components
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
Fix HasDropDown test typo.

"orient" should have been "dropDownPosition", but since the default dropDownPosition
setting is ["below", "above"], we don't really need to override it with ["below"], unless it's for
when the browser window is really short and we want to guarantee the popup goes below?
Fix CssState doc.  this.focused no longer processed.
Refs #214.
clarify KeyNav doc
Remove background:transparent setting for popups.
Not sure why that was there; it seems like the opposite of what we want.
Stop automatically applying overflow:auto CSS to dropdown root node.

The dropdown should handle where the overflow:auto goes because it could
be on a descendant node rather than the root node,
thanks to Scrollable, LinearLayout, flex-box CSS, etc.

Also fixed the popup doc about how the popup indicates it was executed/canceled.
various migration notes, mainly related to changes in 0.4.
Viewport: emit resize event when keyboards shows/hides.
initial work for place's "center" orientation, intended to show dialogs
reposition and resize centered popups when when keyboard shown/hidden, and when viewport changes size