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.
use local require in write method instead of global require