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.