Dropdown align auto controlled props with Downshift

[silviuaavram]

For example, Downshift has highlightedIndex, defaultHighlightedIndex and initialHighlightedIndex. In order to expose highlightedIndex as Dropdown autocontrolled prop, some mapping needs to be done between that and the one in Downshift.

Also, default is something we consider as initial as far as Downshift is concerned.

Follow up from #726

[bmdalex]

Also we should make isOpen and defaultIsOpen auto controlled props as we started using the flag internally in #786

[jurokapsiar]

Closing, according to @silviuavram "we will continue to have different behavior with initial and default in Downshift and Dropdown"