[INTERNAL] m.ComboBox: use a semantic html <button> for the arrow

According to the w3c WAI-ARIA 1.0 Authoring Practices, the ComboBox
arrow button should be an html <button>, or another element with a role
of button.

http://www.w3.org/TR/wai-aria-practices/#combobox

Change-Id: I60c92ec8b23b1dd9b0088b227c24a9caa5d71dcf
[INTERNAL] m.ComboBox: sets "aria-autocomplete" to "both"

Sets "aria-autocomplete" attribute to "both" as more appropriate than
"inline".

http://www.w3.org/TR/wai-aria/complete#aria-autocomplete

Change-Id: I9fb37235f14f6413d23296738d4105797ca29316