OpenMW-CS: Instance drop in scenes
Scenes should accept drops consisting of instance IDs. Such a drop should insert a clone of the instances. If more then one instance ID is dropped at once the position of the instances relative to each other should be maintained, while the centre of the the dropped instances should be used as drop point (the position the mouse is at when the drop happens). Instances should be moved against things behind them in a similar fashion to how objects are dropped. Even in this case the relative position of the dropped instances to each other needs to be maintained.
(RM-3168 from redmine: created on 2016-01-26 by Marc Zinnschlag, , relates #3169)