DND: do nothing, if target tab already in correct private state,
optimization for previous commit (568a599326c57490d8acc69cee2eec38d5d7d97a)
(related to #4)
DND: make new (or target) tab private, if source or target are private
+ rename "dragAndDropUseTargetPrivateState" => "dragAndDropBehavior"
(related to #4)