retain typed context when adding a placement to the session

[ichthyo]

Placements are added by copying into the PlacementIndex. That's an design decision and settles the problem with the "placement identity" -- but creates another quite practical problem: how to pass the more specific type information over this "gap"? Maybe we can completely get around this problem relying on the specific or dedicated session API (as opposed to the generic API which is used to implement the former)? Another solution could be to introduce a trait template to manage the placement target type...