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.