Fix issue introduced by 20950e0f0cbbaa2ed9ddb3306320a69d4691c87d.
Selected items and other invisible items were not ignored by highlightUnderEvent when calculating index.
This caused highlighting of incorrect items when the list contained hidden items.
Merge pull request #768 from bcoughlan/master

Fix issue introduced by 20950e0.
Merge in latest master from select2, includes multiselect/stayOpen issue fix